It's called "marketing to teenagers by hyping a mature title"
We see it all the time, "your mom's gonna hate it" "all the COOL gamers will be playing it" "Best game of E3!" "Not for children!"
We see it for Halo, CoD, BF, and other big FPS games, why? Because all they do is salve the ego-manian that is the young teenager. By playing a mature title and "pwning noobs" they get some sense of entitlement like they've accomplished something. It's sad, but that's what a real gamer is these days. Too mainstream for me...where's my Morrowind Sound and Graphics Editor? *glasses*
It's also a sign of the times, CoD would have never survived if it was a PC only release, because PC gamers were too busy with Half Life (1 or 2?) to notice a brown shooter with no story and a total lack of logical flow.
CoD is aimed at teenagers on a console that love to trash talk. Good job Bill Gates, you proved humanity loves to salve their own ego.