苗哥我查到一個出處，可能能解釋為什麽美國不能用形容詞比如“American":

The Immigration and Nationality Act of 1952 made a distinction between "citizenship" and "nationality" of the United States: all United States citizens are also United States nationals, but not all U.S. nationals are also U.S. citizens.

護照持有者必須是美國公民。而不是所有國籍是"美國的"等同於是美國公民。

如果用American就太寬泛了，把不是美國公民的、或者是不夠格拿美國護照的"美國人"也包括進來了。所以棄用形容詞American？