Remember back in the day when the only way to purchase a certain song off of an album was if it were a released single in addition to the album? Back when digital mp3's werent popular or even heard of. If it wasn't released as a single, tough luck, you have to purchase the whole album. Well, one thing about the New Moon soundtrack is in order to purchase some songs, you have to purchase the album as a whole. The only songs available for single purchases are by Death Cab For Cutie, Anya Marina, and Lykke Li (all signed under Atlantic Records, the distributer of the soundtrack). All other tracks on the soundtrack are sold only through purchase of this album. In a way this is a smart technique, especially marketing lesser known artists/bands featured on the album. In order for someone to purchase Muse's song from the soundtrack, they need to purchase the whole thing, and by default they'll also be purchasing Black Rebel Motorcycle Club and Alexandre Desplat's song from the album as well. So fans who may have never heard of them, will be exposed to new music, and possibly become a new fan of those artists/bands. It's like going to see Death Cab For Cutie at a concert, and seeing an opening act you have never heard of. It's a great way to expose new artists to new fans.
(Note: most of the songs were recorded exclusively for the album, which is one reason why for this).