It's not a guarantee. They get paid for the trailer but to be in the movie as incidental music (on a radio in the background), featured in the movie (montage cut to the music), and on the soundtrack, are three different things. Just all depends on how it falls. I remember when Josh Rouse was in Vanilla Sky, the editor had put it in as a temp track and Cameron Crowe fell in love with it. They offered Josh a "take it or leave it" amount of money and moved on. So who knows what happened in the Features case.
