From what I understand, they can't protect you from anything. It's like someone already said, it's more of a good faith thing than anyone else.
I personally think it'd be easier to slam someone with an obscenity charge if their full blown hardcore site was right there at the root of the directory, than if they at least had a page with a warning on it so people who didn't want to be there (hahaha, who goes to
www.fuckmeintheass.com if they're not looking for porn really?) can have the chance to back out.
Just my opinion though. ;) Ask a lawyer, but I don't think warning pages legally do anything to protect you.