A coffee shop is open, but only if you have money to pay for coffee, you don't look too weird, and you speak the right language. What seems like a public space is actually private, and it might not listen when you speak up calling for change.
Homes and most offices are even more restrictive. They have locks on them and are very selective about who they let in.
Contrast this with a public square, which is open to everyone. Except for people that break a few societal rules, like theft of property, these public spaces are open to everyone.
Case in point: I spent some time on Saturday afternoon enjoying the PROXY square in Hayes Valley. Completely free to linger, and with easy access to food, drink, and music. Sure enough there were hundreds of people who lingered over the course of the afternoon.