I don't disagree with you, but you do have to draw a line somewhere, or every traditionally paid-for app will convert to external registration. Why wouldn't they? It's free money. Still, it's hard to ignore the hypocrisy of allowing Dropbox, Netflix, Kindle, etc without a pathway for sub-billion-dollar companies to follow in their footsteps.
How about a model where the developer pays per logged in user? You'd still have to shake out how much Apple "deserved" for its part in the transaction, eg do subscription apps and top ups get warrant more money?
Edit: Didn't expect to be downvoted, just trying to find models that work for everybody.