When I use to hit the bars, it was the same ones and the group I was with had a rep of tipping really good. As a result they worked to make sure if it was mixed drinks or shots they were strong or very full. I remember on one of my birthdays the bartenders found out it was my BD, when someone ordered a shot it was a triple, to say the least I was wasted in a short time. I use to reff soccer games and was flush with cash on the weekends and I usually only got one beer each night and left a huge tip, even if I bought others drinks, including the bartender, the tips were sometimes 200% +. Not sure about now or where, but the bartenders lived on the tips more than the pay from the owners.
There isn't any real reason that a bartender should do what you say, in that case not leaving a tip would be ok, in my opinion. A bartender or anyone that pulls that stuff needs to find another way of life