I think I heard the Braveheart theme in the Cast Away commercials. What’s going on there? Why would James Horner use a previous theme, especially since it’s reasonably well known. Or, worse, why would some other composer rip off a well known theme for another movie?
I’m glad I’m not the only one bothered by this. If there’s some kind of logical explanation, I’d also like to know what it might be. If the song is wasn’t composed specifically for Braveheart (I don’t know whether it was or not), it’s not quite punishable by death, but if the song was composed for Braveheart, using it in another movie (excluding parodies, etc.) is especially insidious.
This is a common tactic by the film studios. Particularly effective or evocative music from previously successful films are often used in trailers. I was duped by this once. I purchased the soundtrack to a film I hadn’t seen only to discover the music wasn’t in the film at all. Now I pay very careful attention. 
I’ve also noticed that a very small set of music is used - Braveheart and Enya stand out in my mind.
This likely due to the fact that music for a film can’t be scored until the film is very near the end of the production cycle - after the final editing. Otherwise, the music wouldn’t sync with the action taking place on screen.
Oops… I hit submit before I was ready
You have to create the trailer much earlier in the process for marketing and promotion. With just a few choice cuts and some stock music people already associate with movies they enjoyed, you can create a pretty effective trailer that will create buzz among your audience months before the film’s release.
Yeah this bothers me, too. Gives me disturbing images of Forrest Gump in a kilt.
OTOH, Mel on a desert island with tattered clothes is quite a different thing. 
Is it in the actual movie though?
Another possible reason in this case was that the entire island part of CastAway had no score. They did it to enhance the lonely feel, but it probably left them with less music than most films. So they had to add in some extra for the trailer, because the silence would just seem weird there.
Common enough that there’s a web page devoted to listing what the music in trailers really is: http://www.soundtrack.net/trailers/
(They do list it as Braveheart, BTW)
Interesting and somewhat related is that the score doesn’t usually exist even when the movie is being cut together. So the director will just pick existing songs out that fit the mood he or she wants. When they go to the conductor for the score, they’ll often give them this “temp track” to use as “inspiration”. Sometimes they’ll so fall in love with a piece that they’ll actually use the original – which is clearly not cheap. So to relate this back to the OP, the songs from the trailer might also be from the temp track.
Another random interesting fact is that some soundtracks get borrowed from for trailers much more than others, even when the movie they’re from isn’t particularly successful. Wyatt Earp is very popular, as is The Thin Red Line, and The Fifth Element.
You will notice any kiddie/scary movie trailer out there will almost definitely have the soundtrack to Beetlejuice in it.
It appears to be some sort of law.