I can see Amazon using this kind of thing to differentiate better between "real" credit and promotional credit. Like, if I have Prime, and I choose to take a slower shipment time, I can get $1 for MP3 downloads. But that is not the same as if I just had a $1 credit on my Amazon account... it's much less useful. Amazon could make that $1 promotion be a 100 AZC, and it would be a lot more clear.