Are we to pretend all supermarket checkout clerks do an equally good job? No, but we don't tip them. They get paid a wage which is bundled into the price of every item I buy at the store. Same as every other cost of business like their insurance, electric bill, etc.

I've gotten bad service while traveling in the US - how can I not tip? Someone tell me, please, because when I'm paying it's usually before I even get the service and I'm already supposed to pre-pay a 20 percent tip or some such from a checkout prompt that doesn't always let me opt out.

Or when it does let me opt out, it's in plain view of the staff, of course; in a "go ahead and hit 0%, take a chance on whether we'll shit in your food" kind of way.