The law is fuzzy. If a user breaks law while using a service, there's a question of whether the service was a willing participant or abbetor. ToS helps make a case that the service is not a willing participant or abbetor
You also have to enforce the ToS. So Backpage or Craigslist personals are not able to just say, "We're not guilty of aiding prostitution. We tell users not to do that in our ToS."
No, as is the case with Backpage, they could still be held liable if they never actually removed anyone who was engaged in prostitution. It's not enough to say illegal things are not allowed and then look the other way. You have to say illegal things are not allowed, and come down hard on anyone doing those illegal things.
I don't remember the particulars since it was so long ago, but I believe craigslist went so far that they just removed any category that might be used for prostitution. In a court of law, that would be how you demonstrate that you're deserving of liability protection. Backpage was in a significantly different position, because they thought not enforcing anything would give them legal liability. Not smart.
