What makes you think those Mexican laborers wouldn't want to stay in the United States? Why do you think they came here in the first place? Let's think about this: you're a Mexican laborer in California, circa 1942, Mexico inexplicably goes to war with the US. What do you do?
My guess? You start waiving the Stars and Stripes as high as you can, as often as you can in a bid to gain permanent citizenship. You might even volunteer to fight for the United States vs. Mexico in this scenario.
The Americans harshly interned the Japanese for far less, and I presume the pervasive racism of the era would've struck Mexican-Americans as hard as it did the Japanese-Americans.