They make COD so stupid. Launch COD 2, "Welcome to Warzone, the game you didn't buy but we want you to play and buy stuff because fuck you, give me money".
The contemporary Modern Warfare series is a remake of the original series. At least a general new take on a lot from that series. It isn't 1:1 by far.
Instead of letting you launch each game individually, or creating a general launcher that you start and then pick the game you want to play... They chose to force players to launch MW2 as a fake hub, and in that game's main menu, click the MW3 option.
The article says you can tell because apparently if you want to play MW2 you just pick the game type and it starts starving for a match. If you want to play MW3 you have to wait as MW2 shuts down and you wait for MW3 to start, after you already waited for MW2 to start initially.
Activision when you are like me and only care about the campaign:
"You fool, why the fuck would you want to switch to the campaign? Good luck in finding out where we hid that button in this overly convoluted UI. Oh yes, and because you are not planning on forking over additional money to us, we'll restart the game in stingy campaign mode. And no, you cannot be smart and just start the single player executable directly, because we made sure it won't run when not launched through the game we actually want you to play."