The difference is that Facebook is in the market of mining your soul so whatever store they would create it would be targeted to that and the rules and policies will make sure that they are able to reach that goal.

Apple is in the business of selling phones and has decided that a good strategy for them is to protect the privacy of users against data miners. So their store and payment system by default protect users against practices of data collections.

> Having app stores compete for developers and users would be amazing for everyone except the current app store owners

It would be good for developers, not for users. I don’t think a single non-developer user is grasping for having multiple ways to download and install an app, and having to search for multiple stores with multiple payment systems to get one software. For users, the “iPhone” allows to download apps; none of them gives a thought to the fact that it happens via a single “store”.