The reason you got that look is that most people actually do tip the cashier for some reason. When I had a job like that I was always stunned at the kind of tips I would get. The never gave anyone a nasty look for not tipping me though. Occasionally I would come to expect it, but that was only if I was taking their order and they kept making random changes to it as I wrote it down. Even then all their assanin behavior was forgotten for as low as a buck or two.
I have a fairly loose criterion for tipping. At a minimum I require you to refill my drink at least once. At that point I feel you are offering me some service and paying the slightest attention to what I am doing.







