You pay many different types of businesses commission for the service they provide- car salesmen, real estate agents, electronics salesmen. They all earn commission for servicing you by helping you find the product you want. The only difference is that you are in control of the rate of commission instead of letting someone else tell you the commission rate built into the price.
The real difference is that with the car salesmen, real estate agents, and electronics salesmen there is a solid paper trail with those people and their employer paying into the system the rest of us are forced to pay into. Restaurants and their employees are another story entirely.