I don't feel guilt when they swivel that ipad around, more like annoyance. When I'm asked to tip for someone handing me a cup of coffee, I say no tip. We should really be pushing to get away from this relic of slavery, and don't tip unless it's absolutely clear we need to; i.e. at a sit down restaurant.
Ouch, having worked in cafes and restaurants as an undergrad I pretty much always tip 20%. Is it absurd? Of course, but given how badly most waitstaff are paid, the idea that you will somehow eliminate the practice by stiffing a few barristas seems both sadistic and highly unlikely to change the world. Until robots hand me muffins, I accept it as a tax on eating out.
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...