Black diners get shitty treatment at restaurants because of preconceived notions about black people, i.e. black people are poor tippers. So the black diner, miffed at the poor treatment, leaves a poor tip. Or no tip at all. This further fuels the stereotype that Black people are poor tippers, which in turn drives the poor treatment of black patrons at full-service restaurants. Tableside racism is a vicious cycle.