Fundamentally, people are outraged because essentially tipping is subsidizing the wages the employer should be paying, and that's the case even with a restaurant.

The issue is the tipping system. It's counterintuitive but your "generosity" with a tip, once it becomes standard cultural practice, ends up in one way or another getting nullified by the employer. And there's no real way to prevent this as the OP said, money is fungible, so if the employee gets paid more in some way it all just becomes their total earnings in the end, which saves the employer from having to make up that difference because the market price for the employee's time and labor is fixed.