As others have said, seems to be pretty common. I don't ask for it so I've only met one girl who did it, but she didn't offer me initially. She told me that most of the customers who partake in it are Chinese, who are the customers more likely to spend extra money. I wonder whether that's true. I think that would explain why some people never see it. If you are not Asian you are probably not going to be offered, since they also may be afraid of the customer complaining to the shop. Obviously, shops know it, so she wouldn't get more than a slap on the wrist anyway...

That conversation opened my eyes. I now try to avoid Japanese FIFO girls. This is already risky business, I don't need to make it even riskier...