Does it have to do with Ozawa v. US which ruled that Japanese were ineligible for citizenship, and therefore 20 years later, these people weren't citizens and didn't have the same rights as citizens... meanwhile, most German-Americans had been here for centuries and were considered citizens?
Not to discourage more discussion, of course, but you should take a look at this in-depth answer by /u/kieslowskifan to "What was internment of German and Italian aliens like in the U.S. during WWII?", which covers the context for why and how the Japanese-American population (the majority of whom, having been born in the US, were indeed citizens - it was immigrants born in Japan who were ineligible) were treated differently from German and Italian-Americans.