I do not know what Steve said, but the way you state it, "app files" could be the files in the application itself, not user files made by an app. For performance and robustness, they will have more, but they need only one copy of those on their servers, and it could be the one in the App Store. The costs of storing those are a) peanuts, if you are Apple, and b) possibly recouped by the $99 developer fee.