I read where the FBI wanted that kept secret because it was one of the few things that only the abductor knows other than the girl (and the hunters, and the hunters' kids, and the mother). I really don't see the point in keeping it a secret.
There was a reason why and I respect LE's reasoning, even if I don't know what the specifics of it are. It could be that they wanted to keep it a secret in case of similar acts in other crimes. It could easily tie him to other crime scenes.