"[Update]: Microsoft PR informs me that the free access to the map pack was intentional as a limited time thank you after complications with reward tokens. Here are the details:
As an additional reward to those customers that participated in our "14 Day, Buy and Play" program (which ran from Nov. 6 – Nov. 20), we are providing complimentary access to the Crimson Map Pack for a limited time, through Tuesday, December 18 at 9am UTC/GMT.
There were some initial complications with the distribution of the "14 day, Buy and Play" reward tokens, and so we decided to provide the additional reward of complimentary access to the Crimson Map Pack, for a limited time, as a "thank you" to our customers that contributed to the success of the "Halo 4" launch."
"To get what we've never had, we must do what we've never done." - 212 Attitude