You're right that the gratuity is optional, but there is still a strong general expectation that most people will tip. The optionality gives the tipper discreti

You're right that the gratuity is optional, but there is still a strong general expectation that most people will tip. The optionality gives the tipper discretion in any specific situation, not an indefinite free ride with no social consequences. The server wouldn't work for $2.13/hr (presumably) without the expectation, so by never tipping the GP is freeloading on the other customers. Likewise there is an expectation of good service from waitstaff, but they're not obligated to give good service. They too have the option of giving better or worse service at their discretion. So if a server wants to give everyone equal service that's their right. But if I want to develop a reputation as a good tipper to get my service prioritized over GP by servers who differentiate then that's ok too, and rational I think.