Makes loads of sense. If they included ASTs for easy CD-to-console burn purposes, a lot of people wouldn't buy the ASTs they could possibly sell as integrated-into-game DLC.
Plus, the OSTs are the original inspirations, and it's more of a special thing to use them instead.
And lastly, it is 100 songs for "free", with a whole CD of those being the official KoF XIII "Already AST'd" tracks, Plus various other CD quality tracks from the Non-Neo Geo entries.
I like the OSTs well enough. I often switch between them both when given the chance. It's not a huge problem to me.