For the reason I already gave: you can't seriously expect everyone to accede to every demand for paying 20% more. That's not an actual, practical decision theory.
"I can afford 80 cents" isn't responsive.
Then they came for the 20% at Ruth's Chris, and I did not speak out, for I had gotten tipsy on cabernet.
Then they came for the 20% at Olive Garden, and I did not speak out, for I thought the server was attractive.
Then they came for the 20% at Intelligentsia, and I did not speak out, for the creeping norm of paying 20% had been established.
They they came for 20% of my internal organs --- and there was nobody left to speak for me.
The point is that your purported, self-glorifying norm asks one to pay the default PoS tip of everyone who demands or might demand it in the future, to say "yes" to all of them. Counter purchases, grocery stores, fast food, whatever.
Could you maybe consider that there might be better and more practical heuristics?
The countervailing force the iPad is addressing is the fact that customers aren't carrying cash anymore. Without the prompt, counter workers aren't going to get tipped at all.