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.

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?

I feel like tipping is perpetuating the asshole behavior of owners underpaying employees. I actually feel like I'm making the world a slightly worse place when I tip out of guilt or fear of judgment from those around me. But I clearly know I'm in the small minority with this view.

(Purely in the circumstances discussed here. For traditional full service settings, I'd call myself a generous tipper.)

I would argue you're making the world a slightly better place! Your decision to tip (or not) is vaaaaaastly more impactful to the employee on the receiving end than to tipping laws and culture in general.

Yes, the tipped minimum wage enables massive wage theft, etc, but your aggregate decisions about whether or not to tip have approximately zero effect on that.

My sister in law works at Starbucks. She’s a manager so she gets zero share of the tips, but for a busy Starbucks in an affluent area, tips work out to about $100 per week for someone working 35 hours.

Holidays are much higher. Some people will tip for the whole year effectively, putting $20 in the jar.

A lot of rough guesses in the math but $100/week looks like it would work out to 5% at most. Their store is definitely above average for daily revenue.

Most people are definitely not tipping 15% on that muffin.

Starbucks did add tipping to app orders, but it’s a bit more subtle. It’s not right there on the checkout screen before you even receive your items, for example.

"Just give money in every transaction that labels it a tip at PoS. What's the problem? Why do you have to think about whether the demand is merited? I can afford it, and I find it worthwhile to know I didn't break any rules."

Edit: I would say it is indeed unreasonable to expect people to increase payment on any good by 20% merely on the chance that they might be expected to -- few people can afford 20% slack on every purchase. Also, I only brought up VS because I'd seen you question the utility of the concept and this seemed illustrative.

Goods do fluctuate in price day-to-day, but in an uncorrelated fashion; that kind of fluctuation doesn't have the same effect since some are up and others down.