Illegal Alien vs Citizen, does that mean anything in the the US anymore?

If you are doing something illegal, you are generally arrested, punished, or pay a fine.
When an illegal alien is caught for breaking the law, and in the process the officer finds out, that the individual is in this country illegally, why aren’t they deported?
A friend of my was in an auto accident with an illegal alien, and it was his fault. Driving without a license, drinking, crossed the center line and head on crashed into her. They both survived, but her car was totaled. The police did not arrest him or anything. When asked about that, the officer said, they will only get out in the morning, they have no money, no insurance. They will use a public defender, which is another cost to us. It is just easier to let it go. My friend had to pay for her own medical for a bump on her head. She had to purchase another car.