Bartenders provide what could be considered "counter service," and it's traditional (in the US) to tip them. And if you want them not to ignore you when you want another drink, you'd better tip them properly. So when coffee baristas started putting out the tip jar, maybe it wasn't quite as radical as it seemed at first? I find myself tipping baristas now. I tip at food trucks as well, even though the people behind the counter likely own the truck. Like a lot of people, I am ambivalent--no, make that completely confused--about tipping for counter service.
You pay out each drink you order? You don't run a tab? Seems every bartender will ask with the first drink if you want to keep a tab open so they have no idea what you will tip until you leave.