Do you automatically feel obligated to always leave a 20% tip? If mediocre service can generate that much of a tip, what incentivizes somebody to do a good job serving you?
Why tip at a restaurant at all? The workers are making 18 an hour, and they split the tips with the entire staff. Our local Diner charges $35 for two burgers with fries. Breakfast of an omelet, hashbrowns plus 2 eggs, bacon and toast for both is about the same. They drive the regular folks to never go out.