The difference is that Google allows you to sign up for things in apps without using the Play store IAP integration. Apple does not. This is why you cannot buy Kindle books from the Kindle app on iOS, you must go to Amazon's site on your own. On Android the Kindle app includes a full store (not just on Amazon's Android devices).
The google play in-app-purchasing mechanism charges 30%, just like apple does. However, Google play allows you to use your own payment processor for "digital content that may be consumed outside of the app itself", when the app is not a game. Apple requires that all purchases, no matter what, go through their IAP process so apple gets a cut.
