Free speech in America isn't just valued as a legal right before the government, but as a general principle. Most people think that free speech in most situations (not just before the government, and with a few restrictions) benefit everyone.
As a private commercial enterprise, Facebook itself is, for the people who own it, much like my home; they are free to set whatever policy they like regarding what goes on there, as long as they aren't discriminating against certain specifically protected classes of people. It's shortly tricky, it's not unreasonable to argue that a platform the size of Facebook should be treated more like a public space than a private one. But I personally think the bar for overriding freedom of association should be extremely high, and I don't think Facebook is over it. Those who have been disallowed to associate with Facebook remain free to associate with other very similar platforms.
Facebook has been abusing it's massive size to block access to competitors using their Facebook Zero program : https://en.wikipedia.org/wiki/Facebook_Zero
From the page :
> In 2015, researchers evaluating how Facebook Zero shapes information and communication technology use in the developing world found that 11% of Indonesians who said they used Facebook also said they did not use the Internet. 65% of Nigerians, 61% of Indonesians, and 58% of Indians agree with the statement that "Facebook is the Internet".
It is far superior to have bad ideas met with good ideas, than to have bad ideas circulate unopposed within unpopular groups.
HN will ban you if you are unreasonable.
The best subreddits (askhistorians, etc.) will instantly delete low quality posts.
Reddit as a whole banning bad subreddits (fatpeoplehate, coontown) made reddit a better place for everyone, and the trash moved to voat which is barely active now.
Most big sites will now ban you for for threatening or very hateful speech.
Etc.
Your free speech is taken away when something you say lands you in prison. Getting banned from a shitty private website doesn't violate your free speech rights in any way, full stop.