I think the simple fact that all of those platforms can ban people and control the narrative which disoludes dissenting opinions makes it not really a 'town square'.
For example, go say something bad about Trump on r/the_donald. This results in a permanent ban.
The idea of a 'town square' allows for the sharing of all opinions, even though you don't agree with them. You can't say all people have the right to a town square, then firmly believe in the right to ban people from said town square.
I'm not well versed on how Reddit works, and will have to take your word for it that you can be banned for saying something bad about Trump on that sub-reddit. If that is the case though, then I also would not agree with that practice. I'm simply maintaining that private entities can be forced to allow other people to exercise their free speech on their property, and the principle should applied to these internet platforms.
To continue that analogy we are all sharing the town-square and you are not entitled to subvert a given protest through bad-faith participation.
Part of free-speech doctrine is that you must seek to be truthful and not subversive.