Mr. dB said:Tipping may be voluntary, but there's an implied social contract here. Wait staff are paid much less than minimum wage, on the assumption that the balance will be made up in tips. The lowered cost of labor is passed on to the consumer in the form of prices -- if wait staff were paid a living wage, prices in restaurants would be higher.
Therefore, if a customer tips less than 15%, it's a breach of the social contract and an insult. People who cannot comprehend or abide by this social contract should not go to places that have table service.
Great post