It is rude to not tip a bartender.
Joe orders drinks and drinks them very quickly by himself.
Joe wants to finish his drinks.
Joe drinks quickly and tips the bartender a lot for quick service.
The bartender asks why Joe drank so fast, Joe empties his wallet and runs out of the bar.
Joe runs out of the bar, but he comes back later on knowing he'll get good service again.
The bartender counts exactly enough for the drinks.