Walking into a store, generally no problem (no shirt, no shoes, etc), walking into a store and behaving inappropriately = problem and the store can ask you to leave and not come back. For the banned FB users, there was no denial of initial access, but once there, those users repeatedly behaved inappropriately: posting false information (pizza joint as a slavery front), false anti-vaccination information, whatever. You can't refuse to sell a cake to a gay couple but you can ask them to leave if they're knocking stuff over, putting their fingers in the goods, cursing, whatever.

Beheading is illegal. But is it illegal to possess a video of something showing an illegal act committed by another? Robbery? Porch pirates? Assaults? What about gruesome accidents?

I'm kind of a fuddy-duddy, I'm not a porn consumer. But I'm not a prude and have seen enough to question what you mean by 'heavily regulated.'

If Facebook is required to allow free-speech protected porn, why should newspapers be allowed to refuse ads from porn providers? (Not that all do)