If we keep "turning the other cheek" we'll end up with broken jaws. I'm a polite individual but politeness will only get you so far. At some point people have to make a stand and say what is actually on their minds in as powerful a way as they know how.
Being rude also doesn't work as a method of political change? It doesn't, and in addition you alienate people who thing being civilised is a good thing.
Politicians are already one of the most reviled groups around, and that doesn't discourage them.
Stage effective, organised opposition that will actually threaten their plushy careers, then they'll listen.
Also, the organisers invited him because they thought you might want to hear what he has to say, so you do it out of respect for them.