I think Amanda has a good take on the negative reactions Michael Moore gets from people across the political spectrum.
I find people react to Moore the way they act to an outsider. That is, in the sense that I can criticize my family but you can't. But Moore certainly doesn't see himself as an outsider, or an omniscient critic floating above the muck below. Whether one likes his movies or not - your mileage may vary - all of them have clearly been learning experiences for him. His perspective, and ultimate point, always evolves throughout the course of the movie. They're never prewritten find-the-facts-to-fit-the-narrative polemics.