As I understand it, Apple also required any app selling things in iOS to give them 30%. Which is why for example, Amazon does not have an in-app link to their ebook site through the Kindle app. Bitcoin could potentially circumvent this.
If you sell something in your app that is then consumed in the app, you have to do it through Apple's IAP and give them a 30% cut.
On the other hand, if you're selling physical products, you're not allowed to use Apple's IAP. I was actually surprised when I read this, but the review rules did state that last time I looked.
Using an app to manage a payment system that's external to Apple is quite legitimate. That's why the PayPal app exists. And I would wager that Bitcoin apps would fall under the same general category, of being things that happen outside of IAP, and therefore the reason they're getting pulled from the store has to do with something other than the IAP rules.
