It's a tradition, it's how the system works, why would you change that.
That said, I feel like sometimes this is pushed with those tablets with tipping options that present 20% 30% and 35%. Especially in situations where you self serve by going up to the clerk, as a non american it feels like I'm being scammed by a tourist trap, I might press 30% out of feeling that it's customary, but then an American might know the actual rules and either tip 15%, or not tip at all since it isn't customary to tip for self service.
That said, I feel like sometimes this is pushed with those tablets with tipping options that present 20% 30% and 35%. Especially in situations where you self serve by going up to the clerk, as a non american it feels like I'm being scammed by a tourist trap, I might press 30% out of feeling that it's customary, but then an American might know the actual rules and either tip 15%, or not tip at all since it isn't customary to tip for self service.