I definitely think this is not just for Apple products and not even just for this promo. I placed an order for 2 textbooks yesterday that were shipped and then this morning realized that there was a $25 off promo for textbooks on my KSO. I had the promotion sent to my email and then called Amazon to see if they would go ahead and apply the code to yesterday's order and they specifically mentioned the code that was sent to me this morning in my email, so it seems they are checking to see if the codes are matching up with email addresses.
They couldn't apply the $25 credit to the order because it had already shipped but they did put a $25 credit on my account for a future purchase.
I would be wary of buying a code on ebay because of this.