The bottom line is that the guy finds you to be attractive and buying you a drink is the most socially acceptable ice-breaker there is at the bar. I would hate it terribly if a woman accepted a drink I bought her and didn't even say 'Thank You'.

I'm not too up on bar life though, so I won't comment past saying that friends shouldn't be pulling the kind of thing Gruesome's lady friends did. Friends normally won't mind buying you a few drinks if you can't afford it and are honest with them.