I don't exactly have a big sample size, because I usually don't go to this type of bar, but from my experience:
Buying a drink for yourself basically gives you the right to sit in the bar.
If a girl comes over and you want her to stay, or if you call a girl over, you are then supposed to buy her a drink. If she feels she isn't getting enough drinks for her time, then she might go somewhere else. If you aren't interested in any of the girls then that's no problem, they leave you alone.