Third party doctrine for one: the data held by third parties has no expectation of privacy, even if it's about you.

From Wikipedia:

The third-party doctrine is a United States legal doctrine that holds that people who voluntarily give information to third parties—such as banks, phone companies, internet service providers (ISPs), and e-mail servers—have "no reasonable expectation of privacy" in that information. A lack of privacy protection allows the United States government to obtain information from third parties without a legal warrant and without otherwise complying with the Fourth Amendment prohibition against search and seizure without probable cause and a judicial search warrant.

Basically the government's argument: if you wanted it to remain private, you wouldn't have given it to someone else.

I'm reality, it's an area of law that desperately needs to be updated.