The problem with US is that it's so culturaly ingrained that it's really not optional anymore, even if it technically is and no one will call the police on you for not doing it.
As a personal anecdote - when I visited America on a business trip we went to a restaurant where the staff was literally rude to us, the food order was not what we ordered and massively late, and yet our hosts insisted on leaving a 10% tip because
1) low tip will show them they have done a poor job(?!?!?!?!?!)
2) we would look like jerks for not tipping(their explanation).
So basically my takeaway is - if you have to tip even when your experience is poor, or you risk "looking like a jerk", then it's not really optional.
But that's what tipping is here too - it's basically signaling you're not a miser. Sure it's supposed to encourage better service but that's secondary. Just like buying rounds, etc.
As a personal anecdote - when I visited America on a business trip we went to a restaurant where the staff was literally rude to us, the food order was not what we ordered and massively late, and yet our hosts insisted on leaving a 10% tip because
1) low tip will show them they have done a poor job(?!?!?!?!?!)
2) we would look like jerks for not tipping(their explanation).
So basically my takeaway is - if you have to tip even when your experience is poor, or you risk "looking like a jerk", then it's not really optional.