The Twilight Zone sets also had isolated score tracks. It is considered a special feature. It gives the viewer the option to listen to the background music alone without the actor's voices or other sound effects. It is not replacement music.
Here's some more exciting info about the upcoming Thriller set from Tim Lucas' Video WatchBlog http://videowatchdog.blogspot.com/:
I was invited by David J. Schow to participate in recording a couple of audio commentaries for Image Entertainment's release of the complete THRILLER series with...