Sure they can. We've already got a ruleset for what society allows and doesn't, namely the Law. A platform can be neutral up to the point where the law becomes involved, after which it must either capitulate to outside demands or go underground. Any other acquiescence to group instruction is an intentional decision they're making, that they did not have to make. Reddit used to operate like that, everything up to the law was allowed. It's just that doesn't happen nowadays, it's more difficult to say exactly why though.

Is it that other companies make it actually not financially survivable to rebuff the mobs? Or is it more a fear of labels and moderate cuts to bottom line. I'm sure they'd lose some money with controversy but would it really destroy them?