Apparently they don't like fun being poked at the powerful:
http://gizmodo.com/5517890/apple-blocks-pulitzer-prize+winning-cartoonist-from-app-store "[..] we cannot post this version of your iPhone application to the App Store because it contains content that ridicules public figures [..]"
Update: Apparently if you're famous enough / enough people complain, they'll make an exception.
M.