I worked my way through college as a waiter and bartender. My first job was as a golf caddie. I've dried cars at car washes and operated a rickshaw. In all of those cases, there is an understanding that a tip is part of the purchase. More importantly, there is some actual work being done or service rendered for the tip (less so for bartenders, which do get paid above minimum wage).

Buying a muffin at a coffee shop and not paying a premium on top of the asking price is not sadistic. It's a normal commercial transaction. Based on your logic, why don't we tip fast-food workers, grocery store clerks, gas station attendants, etc...