Most postdocs are also immigrants, though not from Mexico and they are not illegal although their visas are temporary ones. I think they could make more in the private sector, but their visas allow academic employment only.
My point (and I'm living it) is that you can make more money, quicker, without government loans for education in a year than most "educated" Americans can conceive. You'll have to sweat, something most academia won't even consider. I have a new recruitment slogan for the high schoolers...."sweat equals money, college equals bills". I realize kids won't get their extra 4 years of indoctrination and beer drinkin' and tail chasing in but.....it is an option. Or they can whine about cheaper import engineers and doctors later......sheesh what the heck has happened to our youth?