Apple wants devs to use in-app purchase to sell things in apps . If you don't they don't get their 30% cut of all sales. It's the same reason Amazon can't sell

Apple wants devs to use in-app purchase to sell things in apps . If you don't they don't get their 30% cut of all sales. It's the same reason Amazon can't sell books in the Kindle app, they have to direct users to the website.