I don't have any sources at hand but basically every existing platform in the past used to charge a fee for releasing on their platform. Nintendo certainly did on their consoles, Windows effectively did by charging for the SDK's. Both Microsoft and Sony also charge for publishing to their stores for the respective consoles.
Apple (and Google) both give the SDK away for free and for a low fee you can publish to the platform and in exchange they handle the payment etc.
30% sounds like a lot but if you consider the cost of having to deal with a payment provider and maintaining it etc it's for most companies a no brainer and peanuts.
