Protesting that workers aren't getting paid by withholding money from workers is a rationalization. In the best outcome, the customer would be paying exactly the same amount that they pay now, or more, it just wouldn't be separated or optional in the bill.

You can tell whether people complain about tipping because of fairness or just because they want a discount by whether they also complain about mandatory, fixed percentage tipping, which is really just a service fee under another name.