I work at an ice cream shop that has one of those tip percentage touch screens. Presenting the customer with the tip screen is embarrassing for me sometimes, especially after a 30 second transaction. But like some folks on here have pointed out, it's not my fault that that's how our manager has it set up.
Also, some transactions can take as much time as a waiter would spend with you at a regular restaurant. If my customer asks for recommendations, allergy information, samples, asks me to spin them a milkshake, etc, I could spend 5 minutes with one person and 10 or 12 minutes with a big family. Asking for a tip feels a lot more reasonable if I've spent several minutes helping you and been friendly while I do it.
A business could pay its employees a base wage of $15+/hr to attract friendly and attentive baristas/cashiers/etc and then pass that cost along to the consumer by raising prices. Or it can pay its employees the slap in the face that is tipped minimum wage in most states, keep its prices supposedly low, and then ask you to tip. It ends up kind of being the same thing.
I get that it feels silly sometimes, and tips have sort of gone too far, but that's how our economy works right now. If you can afford to go out for coffee, a bagel, ice cream, etc instead of eating at home, you can afford to pay 15-20% extra.