Yeah, 15% is way more than credit card processing. It's also worth it, because customers already have an account and credit card ready to spend. And you can sell to kids that got gift cards.
Unless you can convince 85% of potential customers to take the time and (perceived) risk to enter credit card information in an app they've never heard of, you'll still up making *more* money. You can complain about Apple taking "so much money" all the way to the bank …
It's the same reason all those small shops sell on amazon or ebay instead of their own store. It's worth it.
No small dev should even want to go outside the app store for this. That only benefits the big guys …