If the mafia built the street, provided you with a shop to sell out of (with all sorts of nice features, got customers to come to your shop, and acted as a first line of customer support for all billing issues, would it be more reasonable that they got 30%?

How much revenue would developers make if Apple hadn't built the iPhone and the App Store and successfully marketed the thing to millions of customers?

What would be a fair price?

The linking stuff is simply because developers have tried time and time again to game the system by linking to outside shops. Hence the blanket ban.