The glass jar was clearly optional, from an etiquette perspective; nobody will judge you for walking away without dropping anything in it (except perhaps loose change). iPads are different in two ways.
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.