"Tipping is seen in the research on restaurants, as well as in research on gift societies, as a perverse form of gift exchange. Perverse because the "gift" is one-sided. Paules reaches into research on people who hire housekeepers and the origin of small gifts given to maids and other domestic service providors. The ritual was and remains one sided: the powerful gives the gift and the powerless cannot give back."
I think this illustrates rather than contradicts what you're saying.
For Xmas I give the guy who takes out the garbage and vacuums my apt building $100. He in turns gives me a box of See's Candy.
Joanna