Just got lost in your wording.
Couldn't tell you for sure on that but it's an interesting thought. My understanding as with the way we do things with our service men all over the world is that you get a work visa for a limited time and then you have to renew it for an extension. I would guess (key word guess) that the work visas for the us allow the Mexicans to work for a few years and then they need to re-apply, go home, or stay illegally. I'll be in Mexico next week so maybe I can ask while I'm there. This whole thing is more complicated than it appears I'm sure.