The problem is that there are a lot of people in this country who are devotees of Folk Marxism. What is Folk Marxism? It's a mindset that sees all human interaction (whether economic, political, academic, or even social) as a conflict between oppressed and oppressor. The oppressors are the ones with the power (or, I should say *perceived* power), and the oppressed are the ones regarded as less powerful. Therefore, in interactions between blacks and whites, the whites are the oppressors, the blacks the oppressed. Israel, being more powerful, is the oppressor, Hezbollah the oppressed; large corporations the oppressor, consumers the oppressed; and so on. It doesn't matter if any actual coercion or oppression takes place, to the folk marxist, the perceived power imbalance is evidence of oppression and prevents any truly voluntary actions on the part of the oppressed. Hence the idea advanced by some radical feminists that all sex is rape, since the oppressed woman cannot give any meaningful consent. Now I'm not saying that all folk marxists take the ideas to that extreme, but they do look at things through oppressor/oppressed glasses.
In the spirit of America's love for the Underdog, folk marxists make moral judgements based on the oppressor/oppressed category. Oppressors are morally wrong, and anything the oppressed do to an oppressor is morally justified. Hence support for Palestinian suicide bombing of Israeli restaurants and weddings. But realize that the most powerful country in the world is the US. Therefore, to folk marxists, we are also the ultimate oppressor. In any conflict with another country, we must be morally wrong, and the other side is right. Why? Because we have power.
So the knee-jerk reaction probably doesn't spring from a hatred of the US, but rather a simplistic world view that shows why we must always be wrong. Heck, the folk marxists may indeed love the US, but the very prosperity, standard of living, and degree of liberty that makes this country so great also makes it (in the FM view) morally corrupt. I'm sure there are some amongst the folk marxist who get a great deal of self-righteous satisfaction that they are so morally pure that they would condemn even the country they love so well. However, I suspect most are disturbed that they live in a country that (to them) always seems lost in a moral swamp. Probably explains a lot of bitterness on the left.