It wouldn't have been just booing. I'm sure some people would have made plans to walk out en masse as soon as he took the stage (as happened with Elia Kazan). The cameras would have been told to follow the walkout. The implict message that would have gone out to the world is, "the people who know Gibson best don't want to have anything to do with him." He was smart to stay home (and as one poster earlier put it, "count his money.")