Public websites generally have legal protections against their members' actions.
I am sure the owners are mighty busy with all of their other endeavors involved with running bitcoin.
They could not be held responsible in the event that somebody sold some drugs through their forum.
For the same reason that youtube cannot be held responsible for distributing copyrighted content. Even though it's all over youtube.
All you have to do as a website owner is take it down within a "reasonable" amount of time.
That, or host it in Somalia.