Or how about let users buy gift card or apps local currency(trading app). Which could then be used for anything within the app or can be transferred. That way it's no longer a purchase of digital good buy exchange of currency

I would think in that setup you'd have to have it such that you subscribing for another person would not guarantee that another person would subscribe for you. In other words, some people subscribing for others would not receive a subscription themselves, and some people _not_ subscribing for others would still receive a subscription paid for by some other user(s). Basically, you'd pay for a subscription and the benefits would go to some random user that is not you.

Games should ask users to chip in to their Facebook marketing budgets. Cost offset is revenue earned!

I think you just solved a conundrum I was having thinking about this very question -- a video class that you watch in an app is in a sense a purely in-app digital good, and Apple appears to have recently been getting super-aggressive about interpreting it that way. But "not a purely digital good" is a great delineation, and one that Apple should probably be making.

(I'm not arguing that Apple's 30% cut requirement for even purely digital goods is appropriate; I think Ben Thompson's recent suggestion of a difference between goods with marginal costs vs. zero-marginal costs goods would be a better starting point, if there were a way to express it simply. Of course, a better suggestion might just be for Apple to allow developers to integrate their own in-app purchase systems for goods and services that aren't explicitly tied to the Apple ecosystem and use non-Apple infrastructure for delivery -- not just video classes but Netflix subscriptions, Kindle books, and so on.)

Is it really arbitrary, though? The decisions all seem to have reasons, mainly "what is best for Apple's bottom line"