Despite the best efforts of some, it's a free country, so yeah people can tip whatever they want. But if Daddy Warbucks walks in and tips a girl $500 for something everyone else tips $100 for, that's going to cause the going rate to climb. Personally, I'd rather not see that happen.
And let's not pretend an AMP girl is similar to a bartender. Last time I checked, the level of service is much different. A bartender is not going to refuse to make a drink 'cause he doesn't dig the tip.