The "stupid" answer is because it's customary in (much of) the US to tip a bartender, even if that bartender is just serving you a beer. Tipping is both econom

The "stupid" answer is because it's customary in (much of) the US to tip a bartender, even if that bartender is just serving you a beer. Tipping is both economic and customary in nature; you'll find that most people agree that the economics that encourage it are dumb, but that the dumbness of those economics doesn't make changing customs easy.