There may be some technical loop hole that would make vBulletin responsible in the case of a courtcase, if its being used to broadcast illegal material. Remember, when napster first came out a few years ago, the never shared files, only its users. Yet they got sued by the australian government.
Also another possibly explanation is so vBulletin can distance itself from this kind of illegal activity and bad reputation. Open-source, GPL software can be used for any purpose including warez so perhaps this was done to help the profile of vb
Btw thanks for the heads up on that function Nomb