Japanese paid bill at bar

Is this common? I dint know Japanese well and have several times been left with a paid bill after an evening at a bar. I really don't understand how but when I want to leave it is already paid.

Do I now own these people a night out? I'm a bit worried that they may use it for profit next time, although it did not seem so.

I think I might miss something that is very much in the Japanese culture.

I live outside of Tokyo so maybe this is not common in Tokyo.

by Lasrod