The key for retail "Get the feet off the street and into the store". The ipad has the draw to get buyers into the store. If a store sells a 9% margin ipad great - but lets make sure you walk out with a 30%+ margin ipad cover, a 30%+ sale DVD, a 50%+ margin service agreement, and so on and so on.
You think gas stations sell snakes, lotto, drinks, smokes etc because they to offer convenience - the gas gets you in and the rest of the items pays for the lights, heat, etc.