Please tell me you aren't serious.

I think my meaning was quite clear.

A commissioned salesperson is paid on results. The more he sells, the more he makes. It's based on his revenue intake, because that is what he most effectively controls and what he can do to help the company. So, in effect, the customer is paying the employee by way of how much he or she buys.

Waitstaff have much less control over revenue (other than recommending appetizers, sides, and desserts), and their most effective metric is customer satisfaction. Since that's not something the employer can reliably measure (since few people will turn in comment cards or otherwise report on their satisfaction), tipping is one of the most efficient ways to incentivize servers to provide good service.