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.
护照持有者必须是美国公民。而不是所有国籍是"美国的"等同于是美国公民。
如果用American就太宽泛了,把不是美国公民的、或者是不够格拿美国护照的"美国人"也包括进来了。所以弃用形容词American?