- Taking care of payment processing
- Storing the user's credit card information
- Providing a ton of pricing options
- Providing analytics and crash monitoring tools
- Providing deployment tools
- Providing development tools (push notifications, regular updates to their platforms, market segmentation, beta releases, etc)
- Providing distribution
- Creating a market place for the demand
I'm not saying that 30% is the right amount but to say that the stores are doing nothing useful is a bold disregard for the service they are providing.