In a supermarket, shelf space is limited. Online retailers like Apple and Amazon have virtually unlimited shelf space. That's one of the reasons why distributing paid content online is so much cheaper than in print. Compared to print, Apple's 30% take for payment processing, marketing and distribution is a great deal. That's how News Corp can offer a daily newspaper via the App Store for only $1 a week, for which you would normally pay $1 a day.