I'm not familiar with the gent in question, but it seems to me that if it's his program, then the people in attendance will be his followers and supporters, and therefore not particularly receptive to contrary points of view. Nobody loves a good debate more than me, but I don't think I'd be interested in taking on somebody on their home turf, where I'd automatically be dismissed and used as a source of ridicule for people not willing to openly consider anything I say. Doesn't sound like much fun, if you ask me.
This is also why I don't understand why liberals agree to bother dicking with Limbaugh, Hannity or O'Reilly. Even if you're right, you're still held up to be a retard.