Unfortunately, some customers are going in expecting a rub and a release for $40 with no tip, or $10 to $15 tip and thinking they are being generous. So if what you are saying is accurate, the girl is providing a massage hoping for extras and when the client declines, she gets nothing.
I don't see that as being very fair, especially when the girl will then go to the back of the line if there are others working there. She may make $20 off a massage and then wait for hours while being paid nothing hoping that at least one customer upgrades from the basic massage. There are spas that are not as busy as the mainstream ones and a girl may only see 3 or 4 clients a day and not all are tipping $20 or more.