I honestly wish there was a separate app to tell me when (and how much) to tip and when not to tip when in the US. As a European, I'm always baffled by folks expecting tips in weirdest (at least by my standards) of circumstances.
There are some places where people tip merely because there is a tip jar available... usually you just dump small change into there (the coins you get back or maybe a dollar). Starbucks comes to mind: there's no reason to tip at Starbucks other than the fact that there is a tip jar sitting out. If you're not used to tipping culture, ignore it. Same thing with ice cream shops or take-out windows. Absolutely no reason for there to be a tip jar there other than they're trying to guilt you into paying more than the advertised price.
There are certain situations where you might tip for extraordinary service rendered. You don't tip a doorman for opening a door, but you can tip if he calls a taxi for you. You don't have to tip at Starbucks, but if you ordered an extraordinarily complicated drink and they got the order right, a tip is called for. I usually tip even at take-out restaurants if I'm picking up an order late at night while they're getting ready to close the restaurant, or if it's a holiday and I feel bad that they're stuck working.
Some people get really mad if you don't tip a waiter. People rarely get mad if you forget to tip anyone else. So if you're confused, just tip your waiter 10% or 20% of the bill if they brought the food to you and forget any other tipping. If someone complains, just say you're European and aren't clear on the rules.
So not bad at all. It's in no way my responsibility to cover their wages. If the bar wants to raise prices a bit compensate then fine, but tipping is a ridiculous practice.
That's where it gets super confusing though, so a European can pretty safely ignore those and play ignorant.
If in fact they did everything by the book, especially that crucial "check in after 3 minutes / 3 bites following food delivery," and still got less than 15%, then "really mad" is probably warranted given the current situation of a sub-minimum-wage base.
I typically map it out as follows:
Less than 15% = You were nowhere to be found while my food was getting cold and I needed something for the food.
15% = Nothing terrible happened, but maybe we sat there for a loooooong time with menus closed or empty plates.
18% = Normal / everything as expected.
Greater than 18% = You rock. I asked for suggestions and you had great ones, you comped something not related to a bad screwup, etc.
Notice that all of the variation is in a pretty tight range. We could easily normalize it down about 10 points (and reinstate minimum wage) and remap the 4 levels above to <5%, 5%, 8%, >8%.
Do people really say "you suck so I'm going to give you 15% more than the list price"?
Service or entertainment type jobs where you interact with the person for most, if not all, of the time usually get tipped and you're tipping for good service/entertainment. Although some restaurants have this nasty practice of including the tip in the bill...
In the US, most pizza drivers pay for their own gas and the tip covers the gas. Tipping them is a courtesy for delivering your food to you out of their own pocket. They choose to deliver because more often than not the tips pay more than enough to cover gas expenses.
The "interaction" is why you might not tip your plumber but you will tip your hairdresser. You probably aren't making small talk with your plumber the entire time they are working even though they are providing you a service.
Never in my life have I done that. Not once. I tip because otherwise I'm an awful person who took money away the poor hardworking waiter.
It's amazing that the US has managed to become so brainwashed that the idea of mandatory tipping is completely ingrained meanwhile having people in a union (so they can fight to just actually get paid a decent wage) is abhorrent.