Non-renewing subscriptions are consumable. Therefore they cannot be restored. A restore button therefore makes no sense. You also need some kind of authentication/login system for the user. (See below for detailed explanations.)
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There has been some debates in the comments wether non-renewing subscriptions are consumable or not, so I want to say something about it. "Consumable" means that you can consume them multiple times. Like "30 minutes of talking" in a voice-over-IP telephony application. On the other hand, there are non-consumables that you can buy only once. Like when you unlock all levels in a game app. You buy it once, and when you reset the device and redownload the app, you should be able to restore the purchase, so that you don't have to pay twice to unlock all levels. Furthermore, if you don't tap the restore-button in this case but just buy the "unlock all levels" package again, it works, but you will not be charged by apple a second time. That's why it is called non-consumable. It's some kind of metapher. An apple is "consumable". Once it is consumed, it is gone. A chair is non-consumable. You have it as long as you don't destroy it or give it away.
So, it makes sense to regard a non-renewing subscription as non-consumable. If you buy it a second time, you shouldn't pay twice, you should just use the old subscription you already have. If you reset the device, you should be able to restore the subscription once you re-download the app. The restoration is just not done by Apple but by the app itself.
I still regard non-renewing subscriptions as consumable though. I use a simple definition of consumable vs. non-consumable: An in-app-purchase is consumable, when, from the point of view of the StoreKit API, it can be purchased multiple times in the same week by the same user. All consumable IAP-items cannot be restored through the StoreKit. All non-consumable IAP-items can be restored through the StoreKit.
So, the developer is himself responsible for restoring the in-app-purchase of a non-renewing subscription, right? No, sorry. How would the app restore the in-app-purchase of a non-renewing subscription? Suppose I have an iPod and I subscribe to 1 month of listening to the Foo-radio. Now I want to also listen to the Foo-Radio on my iPad. Soo, I install the Foo-App on my iPad and tap the "restore" button. Well... what is the "restore" button supposed to do? How can it know if I already have purchased a "Foo"-subscription or not, and how long it will still be valid? Answer: it can not. This approach does not work.
In order for a non-renewing subscription to work, you have to login the user first, to tie the subscription to some online account. Username/Password, Open-ID, Login via Gmail, Facebook, etc. all would work. Then, when the user purchases an n-r subscription you have to store the fact that he subscribed on some server and link it to his account on the server. You also have to prevent the user from buying the n-r subscription when he is not already logged in. Let's continue with my iPod/iPad-example above. I download the app on my iPad, I login with Facebook, and voila, I can use the "Foo"-subscription now. There is no need for a "restore" button, because the app should check at login-time which subscriptions the user has.
There will be some additional problems to deal with. (1) For example, nothing prevents the user from logging in into 200 devices. Here the problem is not a user with 200 devices, but a university with 1000 students where 180 students share the same account. (2) If the server crashes, some people will probably lose their subscriptions. Problem (1) can potentially lead to decreased income. Problem (2) can lead to angry and unhappy customers.
