If the employer didn't know then they are a victim, not a criminal. The undocumented immigrant tricked them. Punishing someone for unknowingly hiring an illegal immigrant is expecting the impossible. To avoid committing such an act a person would have to have some sort of psychic ability to tell they are being lied to.
The fact is that it is illegal to hire someone who is not in our country legally. I do think that people who are here illegally should be given the chance to become legal citizens. However, hiring someone when you know they are in the country illegally is breaking a law. The penalty should be as it is now.
If I committed a crime I would be prosecuted regardless. This is true whether I did it knowingly or not knowingly. Just because we don't know we are breaking the law does not make it right. Corporations should be held to the same standard as the citizens of the country.