I would bet there were actual differences in performance by the waiters. Why would anyone complain that the waiter is standing near the cash register if they didn't need anything from him? More likely they were attempting to pay their bill and this particular waiter was either lazy or bad at math or had a sour attitude or did something else that people felt was worth complaining about.
Well, @One Brow just stated that this was coming from the employer who said that the waiter was working just as well as the white employees.
Did you miss that part or basically you think they are full of crap?