He's trying to run a scam I think, since the financial transaction runs when they send you the key, not when you register it.
His story is thus: He bought the game at GameStop, and when he put in the key on Impulse it (This is where the story gets stupid) thought it was the key he got from his preorder (Despite that I assume it knows that was the code of a boxed version, since they have serperate SKUs) and then it ran the charge for his preorder, which he should have cancelled anyways.
Sounds kinda rediculous..
Added note, could just be a horrendous glitch that happens by combining a pre-order and a pre-release key, but it seems we would'be heard of this problem before.