I disagree strongly with the action being taken here, but like your parent commenter, I don't get the consternation about their ability to take that action. I think that the first amendment guarantees both freedom of speech and of association to private entities, and that this is no more or less than a private entity exercising those exact rights.

If you want to nationalize all social media and force it to be a public square, that's fine, we can have that conversation. But that's not what people generally argue for, they instead expect private entities to be themselves subject to the requirements of the first amendment. But that isn't how it works. And I think that is a good thing, even when I disagree with the outcome.