So the difference is between "left is a passing lane" and "no passing on the right". In other words, it could be the slow car in the left lane that is breaking the law because it is not passing, just staying there, or it is the other cars that pass on the right that are breaking the law. I am sure the police would rather have both -- ticket those hanging out in the left lane and those passing on the right. Win-win for state/local budget!

There is no law against "passing on the right", at least not if this means "passing someone in a left lane while you're in a legitimate lane to the right of that vehicle".

For example, in the state of NY: Before you pass on the right on multilane roads such as expressways, make sure you check your mirrors, use the proper signals for lane change, and look over your right shoulder for other vehicles. After passing, be sure to check over your left shoulder, and to signal, before returning to the left lane. http://www.nydmv.state.ny.us/dmanual/chapter06-manual.htm