It depends what is effectively a public square.

Having a community moderated by its members is not the same as a huge corporation (which has control over one of a very limited number of effectual public squares) denying a user the right to express an opinion it simply doesn't like - in the same way that (UK) businesses are not allowed to discriminate (e.g. deny gay cake decorations by a religious cake decorator).

Corporations are legal entities in and of themselves, that exist for specific purposes. They are not people.