> wouldn't that violate free speech though?

Free speech can mean two things:

(1) The general philosophical postulate, that society is better when there is a high level of freedom in the exchange of ideas and critique of other's ideas.

(2) One aspect of the above is that government should not censor speech. Like the 1st amendment in USA.

But if most public discourse takes place on forums owned by companies, and the companies start to practice high levels of censorship, then we might formally satisfy (2) but still won't get the cultural benefits of (1).