This is what I leave face down on bad service.
The whole idea behind "tipping" is to leave the your waiter/waitress a little something based on their service to you. If you think it was lousey then you have the right to leave nothing, just as if it was near perfect you can leave up to whatever you wish (I personally have left up to 25% before).
This is what I leave face down on bad service.
Yup. A tip (To Insure Promptness) was originally used before the meal arrived to be sure it arrived at the diner's table when hot (and faster than some other patron's meal who hadn't tipped yet). Eventually it was provided later when settling the check. But to tell me that I must pay it even if my service was terrible? No, I'll lop it off or eat elsewhere. And elsewhere sounds better than this place.