Using your own malformed analogy: The town square is the internet. Twitter is a private shop above subway station with a bunch of garbage that people like to buy. It's super busy. You can scream whatever you want in there, some people outside can hear you. You'll also probably get kicked out if you become obnoxious.
Legalese and dodgy analogies aside the above poster is focussing on the practical effects of these sites being so big and ‘default’.
I get what you're saying here, but the sheer number of idiots "fired for saying X on facebook" makes me think it's not true. It lowers the barrier for saying something and increases your audience reach... but also lowers the mental barrier for saying the wrong thing or someone hypersensitive hearing it.
Have you ever investigated how speech is regulated in other countries? I'm Australian and we have a number of restrictions that make speech 'unfree' in the American sense. But I think they're appropriate and on the whole balanced well.