The US has lots and lots and lots of quality colleges and graduate schools, and I'm assuming these people come over here for their education and just decide they'd rather stay.
I became friendly with a lot of Japanese kids when I went to college. Surprisingly, they don't have very many good colleges even in Japan (that surprised me, anyway), and a lot of them really hated to go back home. Some of them stayed even after they graduated and their student visas expired. They're probably on terrorist watchlists right now because of that, haha.
But yeah, if one graduates with an engineering degree or from medical school, they're not going to have much trouble staying over here legally. Our government doesn't just take "the tired, the poor, the huddled masses" anymore. They take the best and brightest and leave all others to sneak in, pretty much.
My Brain: My friend, My enemy: A blog to chronicle my attempt to recover from anxiety/panic disorderanxietypanicdisorder.blogspot.com/