The employer is to be punished ONLY if he KNOWINGLY hires an illegal... Sort of like knowingly accepting stolen goods.
Well then, they should have to use the same logic that they use to convict people for receiving stolen property. Which is that the person receiving said property for a value severely lower than it's worth should have KNOWN that it was stolen and therefore not accepted it.
Since the businesses hiring illigals are paying them MUCH less than they pay legal workers they KNOW that they are illegal.