I have never understood America's xenophobia and attitude towards immigrants and foreigners, when you consider the entire country consists of immigrants and foreigners.
1 part Projection - Americans rightly fear that those foreigners, once they become many / strong enough, will simply treat Americans exactly like Americans treat everyone who are not Americans but just happens to have something some rich American wants (or just bought a used cellphone from the wrong person)!
1 part Common Sense - If you are an immigrant, having somehow "made it" or at least having a decent time, then it is stupid and counterproductive for you to support the arrival more people like you, who, because they are like you, will be going for the same "living space". Competition is only a nice thing to have when playing a game of sports!
and
1 part History - Mass Immigration didn't work out so well for the American Indians, maybe it will work out the same way for the current American tribes?