It's not a phishing scam. The pack is on the marketplace in preparation for people getting their codes from Halo Wars. It's got some unique coding on it so it'll give the not in your region to everyone.
Its not free to all. It will be free to those who purchase the Halo Wars Collectors Edition. Everyone else will have to pay when the maps are available next week.
Also this can not be found if you go to the xbox site yourself and look for the map pack. Take note the link shown above shows the map pack released on 1/1/0001.
Like previously said, this is only on their as a preperation for the mythic map pack. No matter what unless you have the Halo Wars LE then it will not work for you. Even with the halo wars you would need to activate your coupon anyways.
This happened to all LE exclusive DLC when Xbox Vista was put up.
Be careful if you DO get this on your 360 it might corrupt and if you ever stumble upon a code, the code wont work.
Thats what happened with my Gold Lancer