This seems to be an American behaviour I don't really understand; the inability to separate the things someone's said/written, from the person. If someone writes something you disagree with, why does that invalidate everything else they say?

Michael Moore seems to exemplify this perfectly. He holds strong opinions on a wide variety of (often orthogonal) topics, yet Americans seem to feel the need to either embrace everything he says as good, or be vehemently opposed to him on the basis of their disagreeing with him on one topic. Is it too hard to say "I agree on this" and "I disagree on that"?