Well it seems to me that lots of heresies had this idea at their root way before Catholics were in America. All men had/have liberty as an inalienable right, whether their gubberments recognized this or not — and it seems like it was a shame that the first nation to really get behind this idea was singled out to be the name of a heresy that can basically be boiled down to something as simple as meaning the sin of someone disobeying official Church teaching because they feel like it.
I might be taking this too harshly, and I’m definitely no expert in Church doctrine. It would just hit me the wrong way if sombody said “John Kerry!! What a heretic—he’s such an American.” I mean come on!!
Freegards
Americanism was called that because it was a trend that reached its fullest flowering here, although it was actually a form of modernism that originated, probably, in France (where else?).
There is the well known principle of American exceptionalism, which is not a bad thing in itself, but it did extend in the minds of some American Catholics to their religious life, as well. But an American Catholic is no more permitted than any other to announce that he is free to disobey the moral teachings of the Church because, well, it feels good. So Americanism is actually just a form of European modernism, but the form found in the United States did have a certain component of the “we Americans are special” attitude, hence, the name Americanism.