Interesting points. I'm familiar with the notion that black people don't tip. I recall the instance, in the news, of a Long Island restaurant that added 15% to the checks of black patrons because of that assumption.
How infuriating that must have been!
I drove a cab in LA in the early 90s. In my experience black passengers certainly tipped, as did most Americans. Yes, people from abroad sometimes didn't tip because it wasn't the custom in their own country. And there were people of a particular ethnic group that typically didn't tip, or did so poorly. I'm not going to identify them because my experience was hardly comprehensive & I don't want to start a brouhaha over the subject.
Bottom line here is that service personnel ought to be paid a living wage & the custom of tipping eliminated altogether, save for special & unusual occasions (or if you live in NYC).