How the hell would an employer know? It is illegal to ask about national origin or do anything beyond asking for documents in accordance with the I-9 process. Employers have to walk a tightrope between ensuring illegals aren't working for you and NOT going "too far" in ensuring your employees are legal. Minimum required document check... nothing more.
If you don't see the tight spot employers are in, then you're not an honest person.
So you are in fact claiming that employers don't know when their employees are illegal, and therefore shouldn't be prosecuted?
This is getting good.