Maybe they're going for the same thing as the 501st- If I am correct, (and I'm sure there are 501st members in the 405th to validate this), the 501st was given the "okay" by Lucasfilms to wear their outfits, but not to sell them. So the 405th may be getting the same endorsement.
That, or Microsoft is using a readily made base of outfits for advertising purposes, which seems more likely. There is a bunch of Halo games coming out, so far as I know. Halo Wars especially.