Maybe because over the last century, no country is responsible for more civilian deaths across the globe as the United States. In fact, it isn't even close.
Maybe it's that over the last decade, the US destroyed two entire counties, with civilian death tolls in excess of 300,000 between Iraq and Afghanistan, due to the actions of a group estimated to have 1,500 members? An action that has caused lasting political backlash across Europe, leading the overwhelming majority of British, French, and German citizens to say that they would vote for Obama over Romney because Romney shared a party with Bush Jr.
Maybe its the conflicts around the globe that the US has destabilized a growing nation because of financial or political reasons, including Nicaragua, Panama, Bosnia, Kosovo, and Libya. Or that we financed and paid known terrorists in Afghanistan in the 1980's to fight the evil communists.
Or that the US is the largest Theocracy in the world, without the balls to admit as such? Or that the US was the last first-world country to abolish Slavery? Or that the US is one of the few first-world countries without universal homosexual rights?
There is plenty of reason to hate on the US, especially on the field of battle.