I don't think Kindle is the right comparison for the same reason any other ecommerce site doesn't fit. Apple doesn't 30% everything bought on the Amazon or Best Buy or Newegg app, but an App Store seems like a different beast to me, much more like an in app purchase. Just a thought...
No - Apple only takes 30% for Digital products only.
These apps you mention wouldn't exist because they wouldn't be viable if apple charged 30%.