Actually the forum would be protected by Section 230 of the
Communications Decency Act which basically says that a website with user submitted information (posts) is not considered to be the publisher of such information.
The person who actually posts information (posts a post) is liable (when liability is appropriate) for anything they publish (post).
Any lawyer will tell you that doesn't hold up in court, because it usually doesnt.
Why don't you give some examples of cases when this did not hold up in court?
it won't hold up for a second in court against a lawyer worth their salt, and lets face it, in these types of cases, the prosecuting attorney is always worth their salt.
You asked for proof, so here you go.
One of the most famous cases in internet history.
http://en.wikipedia.org/wiki/The_Pirate_Bay_trial#.22King_Kong.22_defenseSection 230 does not provide protection against a site breaking IP (intellectual property) law, which is what copyright law is.