Best not get too deeply into that "American" word, it rarely makes sense the way it's being used.
Because its usage by many ppl seems to imply it only means US-Americans when the term "American" clearly also includes south americans and canadians, for example.
Well, Kaku looks Japanese too and he's definitely American.
Dafq is 'American'?! To me, it's Native American.
Kaku is Japanese by origin, I guess. So is Nakamura.