Hey, tell that to Captain America. Or Silver Samurai. Or the thousands who turned up to mourn the deaths of Hawkeye, Vision, etc. after Disassembled.
And yes, it's true many Avengers came from foreign countries, such as Scarlet Witch and Quicksilver. But then, so do many Americans.
The Avengers were Americans, if not by birth than by livelyhood. They made public appearances and performed charity gigs for American crowds. Just as recently as New Avengers #13, Japanese criminals are confronted by the Avengers and yell, "American superheroes!? What are you doing here!?"
Whatever their personal origins, the Avengers are viewed by the American public and by the public of the world at large as American Icons, and their actions abroad do reflect on the perception of America's government.