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Sure, I also question the usefulness of withholding that tip, but I also clicked on a link within T.O. Molefe’s essay and found the following, which I think puts the concern in perspective, written in The Daily Vox:

Picking on a white, female, waitress. Really?
While the action cannot be described as racist, it certainly wasn’t righteous, principled or exemplary. Why take on the fight against white supremacy by targeting the lowest common denominator? The waitress, despite her white privilege, may herself be a victim of class and patriarchal structures of power. It seems like an easy target in the face of mammoth social and racial inequality and violence. And is humiliating a white, female waitress really going to chip at the problems our society faces. Where does humiliation fit in with leadership and the quest for dignity for all.
People are still obsessed with white tears
Nonetheless, despite the nature of the incident, the responses to the incident also illustrates how white pain is still seen as more deserving of condemnation and attention. This is white privilege at its finest. Condemnation of the incident has come from all quarters — Even the freaking Daily Mail led with the tears of the waitress. And a Twitter campaign raised R13,000 ($900) as a tip to compensate the waitress.
Let’s take a step back.
Would any of the everyday micro and macro aggressions against black people in this country raise the same level of outrage and condemnation? Instead, black people are told that the aggressions they face are the result of their own incompetence. Can we imagine how much money Twitter would have to raise to compensate the collective pain of black people still reeling from centuries of colonialism?