i think the reason for this is because with the unofficial memory units you can transfer stuff to the pc and back again so there is more threat of cheating gamesave hacking so this way blocking them there basically eliminate the opportunity to do so thus making xbox live a better experience COD was ruined with save hacking.
Thats probably the cover story, but you can do that stuff with other products as well.
Salesman: "Congratulations on your purchase of our new car! Oh by the way, if you put a new stereo in, the car will refuse to start until you put the old one back in. That's just how it is."
yep... big brother is home. they can and want to do more than that. interested in more reading regarding this kind of things ... wikipedia "project palladium" or google: "Trusted Computing' Frequently Asked Questions" or "can you trust your computer".