I agree that if I was the first/second in line I would be miffed -- but taxis in BJ are incredibly fickle, and this unfortunately happens all the time. However, you quite rightly, don't "get it". This is about tipping to thank someone for service, not as so often sadly happens in the US, service personnel acting a certain way to maximise or increase their tip. The cabby's behaviour to jiejie was not motivated by a tip, but it was in turn rewarded by one. Isn't that how it should be in reality, rather an obligatory surcharge, as it is stateside?

tb