The idea isn't "Pay me extra for doing my job" — it's "Pay me a standard tip if I do my job competently, pay me extra if I go above and beyond, or underpay me if I really suck." Think of it as part of the meal's price being contingent on acceptable service, because that's really more what it's like. Viewing it as "extra" is just intentionally working yourself up by categorizing things weirdly.