Why? Because More Is Expected

This is probably so obvious that it is now an unspoken assumption, but let me state it anyway: More is expected.
When someone rhetorically wonders why there are protests against Israel's violence toward Palestinians, but not against PLO/Hamas violence toward Israel, it's because more is expected of Israel. More is expected of the U.S., which is why one objects to any harsh treatment of Gitmo prisoners while accepting Saddam Hussein's sadistic treatment of his own people.

This greater expectation for America, compared to expectations of other empires, is due partly to the historical moment and partly to America's own origins.
Whether we are at the "End of History" or not is debatable, but for Western Europeans, we are at the End of War. Having torn each other to pieces in the last millennium, they are mostly determined to avoid war. In their minds, they never saw no military solution that didn't always end up as something worse.
Post-imperial, post-colonial, almost post-national with the EU, Europe sees intervention, apparent economic exploitation and independent action as a barbaric throwback to ... forty years ago in real time (French colonialism in Algeria ended in 1962), but another age in the European imagination.
Let's be honest about the nature of U.S. multilateralism in this case; while nearly all of Europe's leaders support us in an Iraq war, their populations do not, and this is hardly something for a democracy to cheer. Yes, I expect more from US too.

The formation of the United States of America as an act of will by minds produced by the Enlightenment also creates higher expectations. Founded on principles of guaranteed rights, liberties and democracy, the U.S. began as nearly perfect on paper. Subsequent history mostly has been a matter of expanding those rights, liberties and democracy to all our inhabitants, regardless of race or gender.

America was supposed to ensure freedom of religion, even as England required Irish Catholics to convert to Protestantism, and France required Algerian Muslims to convert to Christianity, to own property and otherwise be treated as citizens.
Thus America draws particular outrage when we are perceived as not respecting Islam. The old empires would have squashed such notions, because they made no promises about religious liberty. But America does.

Despite jeers that we are too new a nation to be sophisticated and wise, we are also too new a nation to have dealt much in the kinds of ugliness from which sophistication and wisdom grow.
Never having been much of a colonizer outside North America, we have not gone through native rebellions that forced us out. Our major colonial possession, the Philippines, had an early rebellion that was crushed (including a massacre of over 900 Filipino men, women and children). Then the U.S. transitioned it to constitutional democracy and eventually granted independence without violence.
Even when we appeared to be going through experiences parallel to those of European nations, we drew different lessons. Vietnam reduced French interest in maintaining faraway colonies, but in America it seems mostly to have taught leaders to go in with overwhelming force instead of the gradual step-up/step- down used by JFK, LBJ and Nixon.

To destroy a nation's existing political structure and replace it with a structure of one's own preference is an act of imperialism. It may be necessary, as it was in post-WWII Japan and Germany, but it is imperialistic nonetheless.

And still, Iraq hawks who think that war will spread democratization across the Middle East have not admitted that a democratic Middle East, at least in the short-term, will put anti-Americanism in charge.

Even pro-American Europeans strive to make the U.S. (aka "Mr. Big" -- a Sex and the City reference?) understand that for democracy to make the world safer for us, the people who are voting must be an America-friendly.

What the administration fails to appreciate is the Spider-Man principle: "With great power comes great responsibility." The bigger Mr. Big gets, the more trust he must inspire in others.