Many of them do, anyway. Let's face it: they do not come for the culture or the political freedom. Or for the weather.
I do not think this applies much to Muslim immigrants to the US, for some reason. The Muslim immigrants I encounter seem to jump into American bourgeois life with zeal and gratitude that can put native-born Americans to shame. Nonetheless, at the moment I am opposed to any new immigration to the US. The country is unsettled by it, and there is not enough work anyway.