This same subject was on another forum, which I will paraphrase (I if legally can), a real Lawyer's response,(at least in his state - CA).
A dinner bill with a percentage gratituity already figured in, is in effect, a written contract between the patron and the resturaunt. Not paying the tip is actually a breech of contract and not necessarily a criminal offense. It could be argued in court, on whether both or neither parties live up to their obligations of the contract.
It may have been incrorrect to arrest the patron, for some type of theft, since they did pay the food portion, of the bill.
Sound's good to me.