No they’re really not. They’re discretionary.
This is particularly why I hate the new custom of tipping before even receiving service or food. You have to do some game theoretic dance to decide whether you’ll get bad service if you tip low up front vs get bad service anyways because they’ve already got your money.
I've not left a tip a handful of times over some exceptionally bad service and food and never felt the desire to return to those restaurants.
Still was interesting they remembered me a year later from a restaurant I only went to once.
Hell you probably weren't the only person not tipping that same night.
I'm kidding about the above, but to be honest, you just don't say anything. The likelihood they're gonna slash your tires or something on your way out isn't even a possibility.
When I was a kid in the 80s/90s, 15% was standard (there was even the mnemonic to calculate it: 10% is easy to calculate, then take half that and add it). 18% was for really outstanding service, 20% was for outstanding service when someone wanted to show off, and 25% was basically unheard of besides some gossip mag detailing how rich a celebrity was ("They tipped 25%!")
Now I see tip amounts that default to 20%. My hypothesis, and perhaps this is a relatively sad commentary in the US, is that the level of inequality in major US cities is off the charts: for many people eating at a nice restaurant in a major city, food, even eating out, is a tiny portion of their overall budget, so adding 5% is really nothing, but people working in restaurants are really struggling to get by (and diners know it), so the increased tip rate is basically a consequence of trying to make unaffordable large cities bearable for workers.