Service Charges in Restaurants. If I go to a restaurant, rather than a self service cafe, I can't avoid "Service". It's a non optional part of the experience. As such the price should be built into the price of the meal. If I get crap service, I simply won't go back to the same place again.
I don't think any other part of the economy has such a strange idea of selling you something that includes a service element. If I take my car into the garage I don't expect to pay an extra 10% because the mechanic did their job properly.