Because they have a product that generates lot's of money to MS through game royalties and Live subscriptions and they know this. Activision has plenty of leverage due to this.
http://www.joystiq.com/2010/11/12/bobby-kotick-on-the-business-of-call-of-duty-dlc-treyarch-inf/
"modest" to them probably amounts to quite a bit of money.
Something about that doesn't add up to me. So because MS has a subscription model, they have to pay 3rd parties while Sony and Ninty don't? Also how is it decided which publisher receives how much money, by their ranking on the most played list? So how much your company earns is based on the current popular fad?
Activision already earns the money from the game and DLC, so unless Kotick blackballed MS by threatening to pull future CoD games off the 360 (doubtful) I'm having a hard time seeing what right Activision would have to that money.