Personally, they should push out the demo earlier to appease to the fans and get a larger hype train rolling and picking up greater steam.
I mean some people actually bought God of War for this, it makes absolutely no sense to have a demo release 2 weeks before the full release of the game, unless it was released for everyone.
When I bought the Ratchet and Clank HD collection I had access to the Sly Cooper: Thieves in Time demo as soon as I picked up my copy. AKA, that was a demo released months before the actual release of the games. Almost 6 months later, Sly Cooper: Theives in time went gold.
Don't know the reasoning behind this, but I have a feeling it has more to do with Naughty Dog than SCEA. I figure more people would have actually bought GoW:A to try the demo if it was out right now and exclusive to the physical edition of GoW:A.