I think you are thinking of the american First Amendment, but just because corporate censorship doesn't fall foul of that, it doesn't mean it isn't a thing. Yeah there is corporate censorship. Not only on the internet, but when even when it comes to traditional media, newspapers, publishing, studios, writers and artists often need to deal with censorship that companies they work for or with imposes upon them.

Considering that private companies control the vast majority internet communications, there isn't even a public social media in the way that there are public physical spaces, and social media companies aren't neutral parties in the way that mail and phone services are, as common carriers.

But I get the sentiment here, that platform owners have the right to control what happens in it in the same way a hosts can set rules of conduct and have the right to kick you out for not following them. But in practice it's pretty messy. Some rules are reasonable for polite coexistence and others are just overbearing whims, or downright prejudice.

LGBTQ people regularly have to deal with their existences being deemed obscene. The most modest images and basic information about them is treated as unnacceptable. They get their posts hid, removed or they get downright banned. You could argue that the owners have the right to do this, but is it a good thing that this happens? Whatever you may think, it is censorship regardless.

At best of times it's difficult to find a balance of how platforms ought to be handled, but unfortunately they aren't always handled with good faith considerations for interests of the people in it. I believe that a lot of people are here because they had to face that.