Why tip at all if they're making $15/hr?
Why tip before they were making $15? The bank teller doesn't get tipped. The cashier at the department store doesn't get tipped. The used car salesman doesn't get tipped. The DMV person doesn't get tipped.
Homemade meals average $2.50. Tonight's was meatloaf, roasted potatoes and peach pie. Can't see wasting gas driving to a restaurant, wasting an hour or so, getting a huge bill and then the waiter expects a tip for merely doing the job he's already getting paid to do and later getting an upset stomach because the cook didn't bother washing his hands.