(Edited: I had some funky quote here that was from a locked thread somewhere else... really... weird...)
Microsoft owns XBox Live and lets its prices.
Microsoft does not own Steam.
Thusly, DLC bought through Steam will not profit Microsoft. By making a deal for "exclusive" DLC, and announcing it as early as the game's launch date, they are essentially trying to 1.) Corner profits, and 2.) Encourage people to buy the XBox version of the game, so that later on down the line, they profit even more.
This makes no sence at all, or I do not understand what you are trying to say. For anyone who bought the 360 version, they wouldn't go through Steam at all. You still have to go to Xbox Live if you want the DLC for your 360. So if the released it at the same time, they wouldn't loose any sales because of Steam or what ever the PS3 version of Xbox Live is called.
I believe it is 360 exclusive is, for people who who can't wait for the DLC will have no other choice to play it on the 360. So you want New Vegas DLC, play on the 360. How many more sales they get, I have no idea, but it must work, since they did it 2 times before that I know of. For Oblivion and Fallout 3. So there must be a huge market of people who can't wait for DLC and must be getting the 360 version for it. Otherwise Microsoft is just wasting money for exclusive rights. Since they are doing it again, it must not have failed for Oblivion or Fallout 3, other wise why would they do it a third time?