Tosh,
Were you reading CITIZEN SOLDIERS by any chance? LOL! I just finished that one myself!
I tried doing some research on your question, but I have found nothing really useful.
I can tell you that during WW2 approximately 23% of the US population had German surnames. I also discovered that some German-Americans were relocated to camps as the Japanese were. As best as I can determine, the basis for these interrments was some affiliation with Nazis, such as a connection with the German-American Bund, the American Nazi Party, etc.
If the percentage of American GIs with German surnames was reflected by the general population...23%...then someone would have to be doing one HELL of a lot of screening...that's a LOT of people. Just my opinion, but I would think that with that many soldiers there would have to be something more than just a German surname to call for more than just cursory investigation. Again, just my opinion.
Steve