To the best of my knowledge, tipping isn’t mandatory in the US with the possible exceptions I listed above in response to YaIrish. Quinn may have some insight with his time in California. I could see that state having a mandatory tip/service charge.
Anyway, while it is not mandatory and there are no consequences for not leaving a tip, it IS expected. Had many friends and family who worked in such jobs in high school and college and they were not happy when someone “stiffed” them on a tip, especially when it’s a good size group and they leave nothing or nearly nothing.
Tipping does not apply to fast food restaurants like McDonalds and such though. Pretty much just places where there are waiters/waitresses.
EDIT: Also tipping waitresses or bartenders in bars is also a thing but there’s less “pressure” to do so so fewer people actually do. And I don’t think people pay much attention to the percentage they give. Just a couple dollars here or there.
EDIT #2. Just realized how WAY off course this got for the thread. Perhaps any more discussion should be taken up in “Ask HMF Anything” thread.