Tipping is a barbaric American custom. Tips have gotten way out of hand.
For decades a 15% tip on the cost of food and drink (before sales tax) was standard, IF service was good.
Now it has crept up to 20% is Expected, and often a mandatory part of the bill. Tips up to 30% on everything including the sales tax are now suggested.
More and more people expect tips. Even at places with no table service where you have to be your own waitress, the sales girls that do nothing except take your money expect tips. Suggested tips up to 30% are shown on the point of sale screen.
Tipping is out of hand. Eliminating the income tax on tips will probable make this worse.