You're not legally obliged to give it - but it's very much expected and you can see the awkwardness when you skip it.
Like just the other month I was in this resort with my family and first night we checked in we went for diner - waiter was very polite, entertained my one year old and gave us tips about the area. We didn't have cash on us so we put it on the room and signed the receipt. Next day he was still professional but not as much effort - then I left a tip for both dinners and suddenly it's back to chat mode, how was your stay, parting drink on the house, please do come back, etc.
In coffee shops you can see the looks you get when you wait for small change.
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.
Like just the other month I was in this resort with my family and first night we checked in we went for diner - waiter was very polite, entertained my one year old and gave us tips about the area. We didn't have cash on us so we put it on the room and signed the receipt. Next day he was still professional but not as much effort - then I left a tip for both dinners and suddenly it's back to chat mode, how was your stay, parting drink on the house, please do come back, etc.
In coffee shops you can see the looks you get when you wait for small change.