The original poster's server required to be disciplined. What she (and others mentioned) have done is the height of rudeness. I would not have missed and hit the wall, and would have made her superior aware of her ignorance. I bet having the minimum wage plus meagre tips is a heck of a lot better than not having an income at all (if you catch my drift).
I agree that people from the UK can sometimes under-tip, but often (as has been said) unintentionally because of the difference in culture - in fact, it goes beyond "difference in cultures" - it is more a difference in the way the service industry workers are salaried. To say it is a difference in cultures, in retrospect, suggests that we are more miserly over here. I think not.
I was in Nashville at Fan Fair (the Country Music Association's Festival) - which was awesome by the way - and when we got drinks in the Sports Bar at the Hilton, we firstly were NEVER charged for any soft drink, and secondly we left maybe $5 for the three of us, maybe a fraction more because the servers were exceedingly nice, and did not ask us for I.D. either (on multiple separate occasions).