It's expected to tip the bartender who serves your drink.
Barry is at the bar with his friends who he invited letting them know it's his treat.
Barry wants to avoid spending too much money at the bar.
Barry sticks to cheaper drinks like pitchers of beer for his friends and tips modestly.
Barry notices how expensive the tab is and decides not to leave a tip.
The bartender is happy to get another tip which helps her daily total.
The bartender remember's Barry as a bad customer who does not tip.