Ok, lets say you have Halo 3 and you pay for all the multiplayer maps. Then you get Halo 3: ODST and you put in disc 2. That contains all of the Halo 3 multiplayer maps including the 3 new ones which you can only get by Halo 3: ODST disc 2, and if you already paid for the multiplayer maps in Halo 3 then you kinda pay for them twice.
Ofcourse you're not paying for them twice if Bungie made disc 1 cost 60$ and then just threw in disc 2 for free.
[Edited on 07.11.2009 5:00 PM PDT]