You could compare it to the street vs a private hall - everyone has the right to walk on the street, but not to automatic admittance to private venues.

Now the problem is that social networks in many respect fill the old function of the 'town square' which was a common area accessible to everyone. That commons area has effectively been divided up in the name of capitalism, in parallel to the Enclosure Acts which destroyed the British commons, so now we have a large number of (private) platforms instead.

There are of course protocols don't suffer from this problem of private capital and monopolistic control, but (like land for the commons) protocols without infrastructure can't compete effectively against platforms' huge network effects. The practical upshot of this is that extremist groups will usually just head for other less discriminating platforms, eg there's a lot of white supremacists on Vkontakte (though I'm not sure how this will pan out if Russia rearranges its internet connectivity with the rest of the world).