Calm down grumpy.
Can you imagine the ruckus that Blizzard would cause if they went to their retailers and said "Cheers for selling the retail copy of our game, try to get it out to people for the 15th if you would chaps. Oh! By the way, we're going to let people that order from us directly play whenever they've downloaded the client." Retailers would be dropping them left right and center.
In your world, who would want to buy a game from a retailer when they're then dependent on waiting for it to be delivered? Everyone in the know would go direct to Blizz, pay a bit more and be playing a week early, leaving physical retailers wondering why they even bothered.
It might be ok in fairy console land where developers can't distribute a game digitally themselves. But in the PC scene, it would be a stupid move by Blizzard to **** off all of the retailers supporting them.
I don't work for Blizzard, and as I said this is pretty common for PC games today. Any Steamworks title is exactly the same as this. Consoles might let you play early, but you're not going to be playing Diablo on a console for a while.