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.
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?
First, they make the experience more closely resemble tipping at a restaurant. Since etiquette fundamentally relies on on implications and expectations, that’s a lot more significant than it would be in other contexts. It creates a real suggestion that you should consider 15-20% mandatory, just as in restaurants.
Second, they force you to make an explicit choice: if you don’t want to tip, you have to actively press a “No Tip” button. This makes it feel more like an active disservice to the worker, rather than just doing the normal thing – to you, and to anyone watching you. Consider if, hypothetically, the worker themselves asked you if you wanted to tip: it’d be very hard to say no to their face! Having a screen prompt you (while the worker may or may not look away) is much less extreme, but it’s still a shift in that direction, compared to an inert tip jar.
Personally, it just feels awkward to me. I don’t mind tipping in restaurants because there’s a clear protocol. I just have to imagine the prices are 20% higher than what’s printed on the menu. But asking for a tip in situations where no tip was previously expected seems almost… passive-aggressive, I guess? If you want to bump prices by 20%, go ahead, I won’t mind, probably won’t even notice. But don’t do what amounts to an optional 20% price increase, creating unnecessary decision and emotional fatigue for me when I’m just trying to make a simple purchase. That is: If I choose not to tip, I’ll feel guilty. If I choose to tip, I’ll feel worried that I overpaid. Both feelings will fade quickly, because the amount is small enough that it’s not really a big deal; but they still damage what should be a simple pleasant experience.