That's right, I remember that one! That bothered me, too.
My theory is they do it keeping in mind all the people who don't pay attention to movie scores (read: the majority). They stick a score from a good movie in a trailer to subliminally convince people that the movie in the trailer is as good as the one they ripped the score from. Of course, Big Fish really was a good movie, but I wonder if it would have sold fewer tickets at first if they'd used a less familiar score in the trailer...