Americans are double-standard hypocrites, that's why.
But pointing fingers exclusively at the US is unfair, for the same can be said about many countries, especially western countries. The USA is just the most obvious example due to its huge influence.
In his well-known book "clash of civilizations", Samuel Huntington states that hypocrisy and double standards are ultimately inevitable side-effects of the west's "universalism":
"Hypocrisy, double standards, and "but nots" are the price of universalist pretensions. Democracy is promoted, but not if it brings Islamic fundamentalists to power; nonproliferation is preached for Iran and Iraq, but not for Israel; free trade is the elixir of economic growth, but not for agriculture; human rights are an issue for China, but not with Saudi Arabia; aggression against oil-owning Kuwaitis is massively repulsed, but not against non-oil-owning Bosnians. Double standards in practice are the unavoidable price of universal standards of principle."
