it sounds like the unlikely outcome of two reasonable policies.
you might not get back the device you send in - say it's a simple broken screen and they're willing to cover it. its easier to just send you an already refurbished identical model and then toss your phone into the queue to be fixed later.
unauthorized parts may violate your warranty and whatever you send in isn't going to get repaired.
They should still just return it. but if you know it's not covered you shouldn't really send it in and it makes sense to cover their ass policy wise even if they do make an effort to just return them.