Because American education and media, even childrens' entertainment, have been blinding Americans to these facts for at least the last 100 years. Superman cartoons had "and the American Way" added late in WW11, until then he (supposedly) just stood for truth and justice.
http://www.archive.org/details/superman_eleventh_hourOne of the saddest lessons of history is this: If we've been bamboozled long enough, we tend to reject any evidence of the bamboozle. We're no longer interested in finding out the truth. The bamboozle has captured us. It is simply too painful to acknowledge -- even to ourselves -- that we've been so credulous: Carl Sagan
We all know about the Vietnam war.
"The only place you and I disagree . . . is with regard to the bombing. You're so goddamned concerned about the civilians, and I (in contrast) don't give a damn. I don't care." . . . "I'd rather use the nuclear bomb. . . Does that bother you? I just want you to think big." : Richard Nixon to Secretary of State Henry Kissinger on the Watergate tapes.
Fewer Americans though understand what happens in the Philippines.
"I went down on my knees and prayed to Almighty God for light and guidance … and one night late it came to me this way.… We could not leave (the Philippines) to themselves--they were unfit for self-government--and they would soon have anarchy and misrule over there worse than Spain's was.… There was nothing left for us to do but take them all and educate the Filipinos, and uplift and Christianize them.": President William McKinley
Our men . . . have killed to exterminate men, women, children, prisoners and captives, active insurgents and suspected people from lads of 10 up.... Our soldiers have pumped salt water into men to "make them talk," and have taken prisoners people who held up their hands and peacefully surrendered, and an hour later. . . stood them on a bridge and shot them down one by one, to drop into the water below and float down, as examples to those who found their bullet-loaded corpses.": Philadelphia Ledger newspaper in 1901, from its Manila
correspondent during the US war with Spain for the control of the Philippines.American policy had learned how to use their interference in foreign countries as a way to further increase the deification of America to Americans.
"If war aims are stated which seem to be solely concerned with Anglo-American imperialism, they will offer little to people in the rest of the world. The interests of other peoples should be stressed. This would have a better propaganda effect." - Private memo from The Council of Foreign Relations to the US State Department, 1941
But there appear to have been moments in history when great leaders foresaw the dangers in this path.
Once a government is committed to the principle of silencing the voice of opposition, it has only one way to go, and that is down the path of increasingly repressive measures, until it becomes a source of terror to all its citizens and creates a country where everyone lives in fear: Harry S. Truman
Nearly all men can stand adversity, but if you want to test a man's character, give him power: Abraham Lincoln
"The liberty of a democracy is not safe if the people tolerate the growth of private power to the point where it becomes stronger than the democratic state itself. That in its essence is fascism — ownership of government by an individual, by a group or any controlling private power.": President Franklin D.Roosevelt
However not all of them opposed it.
"The business of America is business": President Calvin Coolidge