Again: who said anything about violent criminals?
If someone's only crime is that he or she goes to work every day and is underpaid for it, why would you want to put them in prison?
If someone is a violent criminal, citizen or not, throw the book at 'em!
How many undocumented workers do we have? Do you really want to build the prisons we would need to house them? Do you really want to hire all those police to arrest them, and all those guards to confine them?
This makes no sense.