App stores should charge for their service. They aren't making the sales (at least in most cases). They are delivering assets. Charge for the delivery.
I know somebody will start claiming that Apple created the market in the first place. Apple benefits from the large app selection just as much as the developers do - yet the relationship only works one way. Let's delete the App Store and see how well the iPhone does against Android.
Whether this is worth 30% is arguable, but they are not charging for the delivery, they are charging for keeping the garden trimmed.