I don't live in the US, but I have heard of their tipping culture and it's becoming a must every time people will pay for dine in or avail for such services.
But it seems that most workers are expecting tips even if the customers are not comfortable giving that to them and hasn't received good service from them.
It should be out of kindness and voluntarily because they're paid to do their job and whoever started it making it as a responsibility as a customer, well, it's on you folks if you're feeling that you want to give the tip, give it. However, it should not really be a mandatory.
I don't live in the USA but giving tips is also common in my country, it is like rewarding people who do their job more than they are required to do, or if we know for ourselves that they have been outstanding to provide us with a better service we deserve. I usually give tips only when I know I troubled the staff too much with too many question or we know the price that I paid is too low for the item or service done. It is not required but it can help a bit to the staff. It will still depend on us, we are not required to give tip especially if we feel they don't deserve it or the price we already pay for the item is already high to cover their service.