If I have it correct, I as a developer can make a maze application and sell mazes on my own web site, but I must also give users the option of buying them in the app and fork over 30% to Apple. If I do this, it's up to me to convince users that the experience of buying mazes on my web site is superior to the experience of buying them right in the app.
As long as Apple doesn't prohibit applications from "playing" content purchased elsewhere, as a user I have no problem with this, because it gives me another choice for purchasing new content that offers a smooth and easy purchasing experience.
Amazon, et. al. probably hate the idea, but that comes down to two lords arguing over which one gets to exploit the serfs.