Why are purchases on a per-device basis?

My wife is a native Russian speaker and we use this app regularly. However, we purchase subscriptions as we need them and do not renew them automatically. This app DOES NOT transfer purchases from one device to another using the same Apple ID, and even though the app notifies the user that the purchase has been done before, it still charges for the subscriptions instead of transferring to the new device. This is not good for the user and should be fixed immediately

PacoBananas about Russian Television, v1.10