When I've just finished a tasty meal, the last thing I want to worry about is multiplying fractions in my head. The restaurant has a calculator built right into the cash register; shouldn't they be able to calculate the tip if I tell them how good the service was?
This way, everybody wins--the customer doesn't have to fumble with the numbers, and the restaurant might see cash-flow improvements if they put the numbers right there and force people to own up to themselves about how they're tipping.
Besides, smiley faces make everyone's day better, am I right?