Like stock photographers, developers of smart phone apps currently have no way to sell direct.
Android developers can sell direct. Android users are not locked into downloading and installing apps only from the Android marketplace or the Amazon app store.
But if you were an app developer would you prefer to sell a few copies direct to Android users or to potentially sell many more by developing for IOS and being on the Apple app store ? An end user is far more likely to find your product if it is in a store and there is also the perception that content at a store has been vetted to some degree (if only by user review). That's why what Amazon is doing is probably very important for Android as a platform.
It's definitely analagous. I would be far more likely to buy content to use commercially from an agent than direct from someone claiming to be the photographer. An agent potentially provides a layer of validation and authority.
