The reason they tried to do that with AC was to discourage American piracy of the film. Games cant really be shared on the net like movies can (well they can, but not as well), and besides no one person could fan-translate the entire game and get it up in a torrent that quickly.

With AC, almost everyone who could download it did, and those that were disappointed with it most likely wont buy it, while if it had been released on the same day as the Japanese, they most likely would have purchased it.