Well, whatever we might wish, tipping in some situations is customary. It's true that some employees are underpaid, and that tips make a difference to them, but we can never know the details, and it is certainly true that many employees are not underpaid, and still receive tips. We do it because it is customary. Some people try to 'extend' the custom to areas where it doesn't belong, or try to be assertive in ways that do not conform to customary usage, and this can be offensive. In some circumstances, a complaint to the management is justified, but for the most part, a simple refusal should do. One can't be right all the time, and shouldn't expect to be.