Making something illegal doesn't stop some people from doing it, so it's something that should be considered when looking at the overall safety of human drivers. One benefit of robots driving is that they won't get drunk, get distracted because they had a stressful day, get drowsy but still try to make it home, speed because they're late for an appointment, etc...
I don't get it, someone's clearly willing to break the law if they are drunk driving, unless you're banning steering wheels I'm not sure how that's going to help.