When visiting the States I have observed on a couple of occasions where a customer shouted at staff and used the threat of withholding a tip as leverage to be unreasonably nasty to wait staff.
The service industry in the US is awful, and the tipping culture is really toxic. I don't understand those that defend the American approach.
To be clear, I don't defend it. I'm mildly against it. Unfortunately I don't in it's at the root of entitled customer behavior in the States, I think there's a deeper cultural contempt for service workers. You'll see similar behavior towards workers who aren't tipped but who provide some sort of face-to-face service, like cashiers and teachers.