You do know that they do this for lots of big games, right? Dragon Age, ODST, Mass Effect 2, L4D2 - I've gotten and then used credits for all of these. Get Dragon Age credit? Apply to Modern Warfare. Get ODST (which was actually $10 credit and $5 free mp3 downloads)? Apply to Left 4 Dead, etc. You also get it generally for $3 off the top ($56.99) and if you wait a bit in your recommendations they often pop up there and you can lock it at $54.49 many times - plus the $10 credit and preorder bonuses like DA:O and ME2 and ODST content/unlocks.
All with free shipping, same-day as release for most big games (to most addresses). And if you DON'T get it that day they give you $5 off per day - I learned that with ODST due to a flood in texas or wherever the shipping station was. Wasn't even their fault.
And NO tax.
It's not a FFXIII thing. It's an "Amazon wants to be your first choice for gaming" thing.
I bought about 40 games in the last 12 months from them. Highly recommended.
EDIT: and whenever you lock in you get the best deal/lowest price/all pre-order bonuses if it changes. I pre-ordered Halo:Reach when it was still listed as Halo 4 or something, and my price is listed at $54.99. So no matter what price it ends up at, I can't pay more than that. If it drops lower somehow, I'd get that too.