It is not KISS from the engineers perspective, but from the user perspective. >> The best interface is a simple interface, not a whole bunch of programming voodoo to achieve a simple task.

The point is: the best interface is NO interface. Go to car. Open door. Sit. Not: Go to car. Find keys. Unlock. Open Door. Sit.

Ordering your food is less simple than ordering your food and handle money? But you do loose a lot of anonymity, which offsets the ease of use.