That's conflating two different problems. The issue of someone posting something without permission is one, and what Facebook does with what it distributes is another, entirely different second issue. We are discussing the second.

Is it dystopian to regulate that sites cannot host and distribute pedophile content for instance ? Regulations are needed, in the internet as much as anywhere else.

Just think of the real world analogue. I give you access to my stuff. Maybe I lend it to you, maybe I let you crash at my place. That doesn't let you do whatever you want. If I lend you my car and you take it to a car crusher, that's messed up and regulations protect it.