I know this post hasn't been touched in a while, but I didn't think anyone touched on one of the reasons that music from other films get used in trailers.
Generally, studios/producers "own" the rights to the music that is used in films (they are, in effect, the ones who paid for the music).
Trailers very rarely use the actual music from the film being previewed, because in 99.99% of cases, the music hasn't been written yet, or is barely off the drawing board. (film scores are quite regularly tossed off in a matter of weeks, exceptionally months, and EXCEEDINGLY exceptionally, for Lord of the Rings, MANY months or even years).
So the people who create trailers try to find music that is in the studio archives which matches the advertising angle they are using for their film.
Notice, for example, how often the music to Cocoon (James Horner) comes around for scenes that are meant to be grandiose and "mystical".
Very few Hollywood composers retain absolute control over the music they write for film productions. I think you could probably count them on the fingers of one hand.