Has Apple stated its view on legality of apps using alternative electronic payment systems?
As I read here Google does not allow 3th party merchanting services:
https://stackoverflow.com/questions/10217727/apple-android-3rd-party-merchant-services

I'm quite sure that Apple wouldn't allow them either.

Although, by law, Bitcoin is not considered as a payment, since law does not recognize Bitcoin as a currency.

So by the rules i don't think they can ban you for using Bitcoin, but as we all know, Apple just decides itself what it allows and what not.