I've lived here my entire life and I can tell you it's definitely not this simple. I can think of a dozen examples where it was ambiguous what to do: the first taxi I took, the guy who came to repair our washing machine when I was home alone as a kid, the guy upthread here claiming that 20% is the "widely-accepted minimum", someone else on this thread talking about how a waitress complained that he never tipped on his takeout orders (you're supposed to tip on takeout orders??). It's a fundamentally idiotic system and I don't have trouble understanding at all why people like GP would find it unpleasant as a traveler.