Dang… That's so wrong. No one should be doing that type of stuff to anyone. Again, I'm still shocked about that one convention re-invited Vic after they canceled him.
This is something I -can- answer on. It has to do with his contract that had been already signed. The convention could have been sued for breach of contract had they not honored the invite and as I am assuming, they might not have included a morality clause or clause to give them an out to effectively back out. (They may have, I haven't seen it. This is going off of my knowledge of Guest contracts.) The guests that pulled out have cited their own reasons and the con agreed to let them out without penalty, so that was a mutual break.
The very least that people who are attending can do is patron to the artists who are advertising their profit donations going to RAINN and/or avoid the man and his fans.