I think I get it. We're trying to help the other countries build stronger economies. When people are desperate, feeling powerless and without hope, they tend to seek violent means to attain that power and hope. But by empowering other countries to lift themselves out of poverty or giving them the tools to become "first world" players, we actually make ourselves (US) more secure.

Said another way, it's a form of foreign aid. Looked at from this point of view, you wouldn't let anyone stay unless there was really a special need.

Academia needs its army of minions to teach undergrads and do the grunt research work. Fewer and fewer Americans are willing to go through the grind that is grad school in Science or Engineering. Thus, one must find cheap labor abroad.

Why are they kicked out afterwards? Maybe because most of them are no longer necessary? Skills such as writing research papers and grant proposal are not in high demand in industry...