I was wondering the same thing, myself.
Maybe (and this is just about the ONLY thing I can think of), they're throwing the Mature label on Book of Exalted Deeds because they plan on getting into overtly religious topics? Rules and stats for real-world religious institutions, and whatnot? Maybe they hope to avert some of the inevitable outcry, and maintain a sense of seriousness, by saying "this is for responsible and mature readers only"?
That, or BoVD was labelled Mature, and it had good slaes, so they want to keep the bandwagon going. They throw in some rules on celestial/mortal nookie, stamp a label on the cover, and call it a day.