I was under the impression that a lot of immigrants join the military as a route to a free education, if nothing else. There have been bills filed in Congress to make attaining citizenship easier for non-citizens (but legal residents) who are enlisted and serving in Iraq and Afghanistan. I think it's more for people who come here as children than for adult immigrants, though.
It would be nice if everyone who joined the military did so out of patriotism but for most people it is, at best, only part of the equation. The military provides a great way out of poverty for people who are willing to work hard.
I served for 27 years in the NAVY. We had many Philipino nationals who were in. Not anymore.
My ire is raised to see these "protestors" waving their flag and now ours yet haven't the slightest idea who gave them that right. Their ability to protest, close our streets and thumb their nose at our laws is because of people like myself my son Lt. USMC and those serving now and in the past. If it weren't for the military they'd be rounded up and shot just like in IWAAACKIland.