You are confusing freedom of speech with "the right to freedom of speech". Me restricting your freedom of speech is still restricting your freedom of speech, it is just legal and does not violate the first amendment. The government restricting your freedom of speech is not legal, and violates the first amendment.

You're right, but it does show an endemic lack of respect for the concept of free speech.

No, it very definitely limits freedom of speech, it just fails to infringe the constitution.

And a "privately-owned platform" that is open to the public, used by a nontrivial amount of people, advertised by the owners as a place for public discussion, quoted in the media, etc etc etc isn't so "private" anymore.

Freedom is not about the law.

Except online, all platforms are privately owned. Hence you often get de facto censorship. You can technically say something not allowed on those platforms, but then find there's no way to draw any attention to this content/those views. It's like living in a company town where anything the administration don't like isn't allowed.

It's a problem with freedom of speech and private property in general, and why some people have considered the idea of treating large social platforms as closer to utilities rather than businesses.