A tip is a discretionary bonus to reward good performance. The first part is discretionary, the second part is good performance. If you hold customers hostage with poor performance, customers ought to have an option to decrease your pay, in theory. But it is in theory only because it is asymmetrical, they have an option to increase your pay but not decrease your pay because workers are protected. And then, back to practical, normal performance = no need to tip, doing what is expected.