We have anti-discrimination laws on the books, which allow a person who is denied employment on the basis of race...etc... to sue the employer over it. Why not have a law that allows people who are denied employment in favor of hiring an illegal immigrant to do the same?
Technically, it would be a pro-discrimination law. (Requiring discrimination against undocumented illegal aliens.)
The current laws we have on the books just don't seem to be cutting it. Sure it's illegal to hire someone without proper paperwork, but if you do, it's not like anything is going to happen. You only have to make a very superficial effort at verifying their documents and it becomes very hard for the state to prove intent. If a few of these employers get sued by private citizens, on the other hand, and had to pay out some settlements.... I think they'd start shaping up their act pretty quick.