One small township near us does this. They passed a law that states restaurants must put an additional 2% charge on every bill that is used to pay "kitchen staff." When we asked our server about this surprise charge they said "the law requires that we charge it now."

We decided to simply reduce our normal 20% tip by 2% and pay that. We don't like the fact that the city has stepped in to mandate additional fees like this and think this practice should stop.