Allerdings kann ich in anderen Apps sehr wohl In-App-Käufe geräteübergreifend wiederherstellen. Das liegt definitiv nicht an Apple.

Auf deutsch heißt das jetzt: keine Chance oder was?

In-App Purchase Types
Items offered via In-App Purchase fall within one of the five following purchase types. Make sure to evaluate what the proper purchase type is for your item before you set up your In-App Purchase product in iTunes Connect.

Consumables
Consumables are In-App Purchases that must be purchased each time the user needs that item. Consumable In-App Purchases are available for both iOS and OS X apps.
!Some examples of Consumable purchase types are:
• Supplies in a game (ammunition, health points, cheats, extra lives, etc) !
• Accelerators used to decrease advancement time within an app

Non-Consumables
Non-Consumables are In-App Purchases that only need to be purchased once by the user and are available to all devices registered to a user. This purchase type is used for services that do not expire. Non-Consumable In-App Purchases are available for both iOS and OS X apps.
!Non-Consumable content may be bundled in your app’s binary when you submit to the app store, or it may be downloaded after the user makes the purchase. Starting with iOS 6.0 and OS X Mountain Lion 10.8 your content can now be hosted on Apple’s servers. This eliminates the need for you to maintain a server to host content. There is no additional charge for using this functionality. After the user makes a purchase, you can use the SKDownload class in Store Kit to download the content to the user’s device. Hosted content is built using Xcode and uploaded to iTunes Connect. Hosted content is particularly useful if you have large content to download, but it can be used for any downloadable content. See the iTunes Connect Developer Guide for details on how to configure hosted content in iTunes Connect, and the SKDownload class reference for details on how to download the content using Store Kit.
!Some examples of Non-Consumable purchase types are:
• Additional levels within a game
• Access to pro-features such as additional camera lenses or audio effects • Lifetime subscription to a service
! • Books and individual magazine issues
!Auto-Renewable Subscriptions
Auto-Renewable Subscriptions allow the user to purchase episodic content or access to dynamic digital content for a set duration time. At the end of each duration, the subscription will renew itself, until a user opts out. The Auto-Renewable Subscription In-App Purchase type is available for both iOS and OS X apps.