Usually the price for something is tied specifically to the thing, but we do not buy food in a restaurant that way. Instead, part of the price is tied to the food and the remainder is given freely to the waiter. In other words, so long as the waiter's service is acceptable, the tip is his and if you don't give it, you're taking his service without paying for it. I don't like this system, but that's our system here. To avoid it, stick with fast food joints.