Employment can not be stolen. Its not property nor it is anyone else's entitlement.
Surely you're not going to argue that everyone in the world has the right to work in the United States?
If someone with no right to work takes unlawful employment, they have stolen it.
I don't claim to understand a great deal about economics, but I do understand this much: if a person has a business and he needs help running it, he has to hire someone. Let's completely take the race issue out of it. Lets say you have Paco, an illegal, who will help you for $3 an hour, and you have Manuel, a second generation American citizen, who requires $5 an hour. If you hire Paco, you have committed a crime, you have aided Paco in HIS crime, and you have cheated Manuel out of a job, so, in that respect, Paco has stolen Manuel's job.
Now, if you had the same kind of dollar discrepancy, and both Paco and Manuel were American citizens, then you'd have a fair market argument, but the employer cannot use that argument to justify breaking the law.