They don't assume anything, they've been told how to interpret it so that's what they do.

Since you like reading law review articles start here, and I've copied some excerpts to save you some trouble.

https://scholarship.law.stjohns.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=1206&context=jcred

In [Professor Eugene] Volokh's argument [for a broad individual right], the operative right in the Second Amendment is "the right of the people to keep and bear arms" and the justification of the right is to provide for a militia, "being necessary to the security of a free state." A facial construction of these clauses would be that a right should be no broader than its justification; thus, individuals have a right to bear arms only to the extent that it is related to a militia or defense of a state. Or, as Volokh sets forth the issue, "[s]ome argue that justification clause should be read as a condition on the operative clause: The right to keep and bear arms is protected only when it contributes to a well-regulated militia or only when the well-regulated militia is necessary to the security of a free State." Volokh's response to the question flows from his review of state constitutional provisions. Although rarely occurring in the federal Constitution, state constitutions often contain justification clauses. Volokh explains that there need be no exact fit between the right and the justification: "one should expect the possibility of a mismatch between justification clause and operative clauses: The means chosen to serve the end will often be somewhat broader or narrower than the end itself. But it's the means that are being made into law." In Volokh's words, the justification clause does not "trump the meaning of the operative clause...." Thus, there may be no law to "deprive people the right to keep and bear arms, even if their keeping and bearing arms in a particular instanne doesn't further the Amendment's purposes." Volokh has made a convincing case that the breadth of a right may exceed its justification. It is less convincing that this premise compels the conclusion he asserts. The questionable aspect of his analysis is the breakdown of the Amendment into operative and justification clauses. It is clear that the "right of the people to keep and bear arms" is justified by the need for "security of a free state"; but to which clause does the militia belong? Only if the militia belongs to the justification clause may the right of the people be broader than participation in a militia or acting for the preservation of a state. If the militia belongs to the operative clause, Volokh's conclusion does not follow. Under this reading, the individual right to own guns would be constrained by participation in a militia because the limitation occurs in the operative clause. The broad rights advocates would then be reduced to arguing the logical absurdity that the individual right is broader than itself. Should the reference to the militia be construed as belonging to the justification or the operation of the Second Amendment? It is more likely that it belongs to the operative clause. The militia has no independent justification or reason to exist. Its function is strictly in subservience to larger ends; in this context, it exists to protect the security of the state. It fulfills this function by operating as the tool through which armed individuals come to the aid of the state. The operative right should thus be read as a conjunction of the right to bear clause and the militia clause: the right belongs to individuals in a militia. Under Volokh's analysis, it is possible that individuals in a militia have rights broader than relate to the security of a state. It is not possible, however, that there is any constitutionally protected individual right to bear arms outside of a militia. To read the Amendment in this manner would require not that the right is broader than the justification but that the right is broader than itself. Thus, Volokh's argument collapses for failure to identify the militia as belonging to the operative clause of the Amendment.

Wow, you don't often see an argument from a scholar as widely respected as Volohk--with whom you must be familiar as a fan of law review articles (he wrote the book on how to write them)--be absolutely torn apart with irrefutable logic.

.....

It is also undisputed that the Bill of Rights created no new rights. That there was no right of individual possession of arms for private purposes before this document voids any pretension that such a right existed after the document. The Bill of Rights was meaningful because it guaranteed that Protestants would not be treated unequally compared to Catholics in terms of possession of arms. It also transferred control of weapons law to the Parliament so that the English militia would never again be the tool of royal machinations. But the document also codified the central features of possession of arms in the country: arms were primarily important as tools of collective safety, and they were within reach of the law to regulate. The subsequent history of England shows beyond peradventure that there was no private right to firearms. The American colonies put great emphasis on the militia. This was primarily a function of the strong historic aversion against standing armies. The aversion intensified during British occupation of the colonies. But in again the historical record is devoid of any suggestion of an individual right to bear arms outside a military function. This is shown in the original state constitutions, not one of which unambiguously recognizes such an individual right.

.....

The last refuge of the gun proponent pertains to the issue of self-defense. This is certainly a major perceived reason for the private ownership of guns. In a 1979 survey, when asked why they possessed a gun, 20% of all gun owners and 40% of handgun owners cited self-defense as the reason. It is unfortunate that these people may be operating under a delusion, having subjected themselves and their families to great danger in the guise of self-protection. One study examined the number of times a gun is used in self-defense against the risk of having a gun in the home in King County, Washington. The risks measured by the authors were the cumulation of "death from unintentional gunshot wounds, homicide during domestic quarrels, and the ready availability of an immediate, highly lethal means of suicide." The authors conclude that for every instance of a death resulting from defensive use of a gun, there were 43 gun deaths resulting from domestic fights, accidents, or suicides.

One researcher, commenting on the study, noted that "the justifiable use of firearms for self-protection is a rare occurrence and carries with it much greater associated risks of the death of someone other than the perpetrator. The same approximate result obtains on a nationwide scale. In 1992, there were 308 justifiable firearm homicides in self-defensive compared to 15,377 total firearm homicides. Surely, no public policy can be sustained when the negative consequences occur 50 times more often than the positive consequences.

...

There was never a single mention at the Constitutional convention about an individual right to bear arms.

during the ratification hearings on the Bill of Rights in Congress, a draft of the Second Amendment was originally introduced which set forth an individual right to bear arms (that is, which did not attach a qualifying militia clause to the clause setting forth the right to bear arms). However, this version, which would clearly have set forth an individual right to bear arms was soundly defeated, and anew version, written by Madison, and which qualified the right to bear arms to its use in the service of a militia, was subsequently adopted and incorporated in to the Bill of Rights.

Wow we could have had it written right in there, but that version was soundly defeated because everyone there agreed it would be idiotic to allow any random person to buy whatever guns they want.

....

The U.S. Supreme Court in Miller stated that "The Second Amendment guarantees no rights to keep and bear a firearm that does not have some reasonable relationship to the preservation or efficiency of a well-regulated militia." There are two possible interpretation of this language. The first is that the Second Amendment gives every citizen a right to possess a weapon which might conceivable be used for military purposes. The problem with this interpretation is twofold: first, it leads to the remarkable conclusion that citizens have a right to possess such military weapons as machine-guns; bazookas, and perhaps even suitcase-sized nuclear weapons, but no right to carry a weapon such as a Saturday night special, which no branch of the military has ever issued to its troops. (Even the gun lobby has never suggested that there is no right under the Second Amendment to carry small handguns such as Saturday night specials). The second problem with this interpretation is that every circuit court since Miller, without exception, has rejected this interpretation of Miler.

Hey, until we got some illegitimate Supreme Court justices who were willing to pedal the same lies that you got tricked by. Now anyone can have any gun anyone wants and all gun laws are unconstitutional because "reasons."

The author of that law review article also rewrote history.