When I pay my developer fee every year, as far as I see it, I have paid Apple for the use of their IP/Tools/Platform. Maybe that isn't exactly the right way to look at it, but that is the way I see it. If you are making money in your app, Apple is providing the means that you are able to process payments, and also providing you with the tools and platform to manage those payments, collect your money, manage your app promotions, and if your app is popular, advertise for you. If I don't make money for my app, i'm not using those other benefits that they provide so I shouldn't have to pay for them. It's like a base rate to play in the store, but you have to pay more to use more advanced features.