I believe the law states that the police are allowed to ask for 1 of 4 forms of ID (Drivers license being one of them) to prove you're legal, if you are stopped for any legitimate reason. The law doesn't say they can just go around picking on Mexican people at random. I'm not quite sure why everybody's in an uproar about it. They already ask for ID, the difference being, I believe they can act on it now.