I'm told by Blizzard PR representative Bob Colayco that the rating was based upon an early build of the game and Blizzard has already appealed the rating, submitting a new build to "address the Game Ratings Boardâ€™s concerns."
The game was rated suitable for those over the age of 15 last year based upon a build submitted at that time. I'm told by lead producer Chris Sigaty that this should not delay the game beyond its targeted mid-year release.
One might speculate that the adults-only rating was rendered with influence from KeSPA, the organization that oversees StarCraft: Brood War eSports in South Korea. KeSPA and Blizzard are locked in conflict over StarCraft II in Korea as a lot of money stands to be made with a transition to the sequel.