Racism

In WW2 the Americans interned all American Japanese people, even if they were third generation Americans.

All you had to have was 1/16th Japanese genes.

No such ruling for German or Italian descendants existed despite the US being at war with Germany and Italy.

Had they tried, they would have run out of Americans to run the concentration camps.

Basically, whiteness functioned as a presumption of loyalty unless proven otherwise, while non white ancestry functioned as a presumption of threat