Good point, as many poster's before have stated, they buy into the US Economy, as well as pay taxes on their income (unless that's off the books) and they pay rent which includes property tax.
Sure, a lot send money home, but hey, wouldn't you do the same?
If you have a family to feed, you're not going to sit on your ass and be sorry for yourself and say "oh, it's too hard to get a job in America legally, so I'll just let my family starve." I'm sure most people do what it takes, and if the law says otherwise, then so be it.
We're all humans first and Americans second.