It's all the damn buttons. When the US got the same chip technology that Europe had, and all those little portable machines became commonplace here, they had to put tip buttons on all of them, and then everyone started using them for everything.
The only time you should feel obligated to tip in the US is in a sit-down restaurant, where the waiter takes your order and brings you your food. If you sit through the whole meal and don't leave a tip, the waiter will think they've done something wrong, as some of their hourly pay is expected to be covered by tips. But no matter what the buttons say, 15% is sufficient, and 20% is generous.
Getting takeout from the same place? Tips are not necessary. I sometimes leave 5% if I like the place. But since there's no 5% button, I take the time to do the mental math and enter the amount whe paying.
Fast food? A concessions stand at a ballgame? You are under zero obligation to leave any tip, no matter what the buttons say.