Not parent commenter, but the common argument is that for the class of service jobs where the employee's role is to interface with the customer and provide them a service, customers are in a better position to assess the employee's job performance than a manager. For that class of jobs, giving customers influence over the employee's "merit bonuses" is a more accurate system of rewarding performance and requires less managerial oversight to ensure good service.

There seems to be some logic to that to me. Whether that justifies a reduction in the minimum base wage for tipped roles, etc. I think of as a separate question, but I don't see tipping as a structurally problematic norm as long as the job interacts with many customers and a significant portion of the population actually tips.