>It’s no different from VAT versus sales tax: same result, different math.

There's lots of evidence that tips vary significantly based on the traits of the customer (like the customer's self-esteem and sense of shame: https://www.emerald.com/insight/content/doi/10.1108/ijchm-02...) and the employee asking for the tip (e.g. attractiveness and simple demographic characteristics: https://www.sciencedirect.com/science/article/abs/pii/S01674...).