Websites don't need to be complete anarchist societies; I think many social media sites are morally obligated to follow up on their ToS and not just let everything go to shit because their AI thought what is clearly a credible terrorist threat is perfectly fine (this has actually happened to me on Facebook).

That's true, and individual sites have the right to enforce whatever rules they like, but that doesn't extend to the platform as a whole, because the internet can't be made to conform to any specific moral or legal ideal. Wherever you have freedom of speech and freedom to communicate, you also have freedom to conspire. And, as connections on the internet are necessarily nonlocal, someone without an account on a heavily moderated platform can still threaten you depending on the information you leave publicly available.

It was once said that the internet interprets censorship as damage and routes around it. This turns out not to be as true as once assumed, because of the network effect of social media sites, but it is still true enough, where "censorship" is considered as a politically, socially and morally agnostic term. Push the racists and trolls off of Twitter, they'll just go somewhere else. That's intended to be a feature, but only because it applies to everyone.