To keep the Yeerks from finding out everything. To keep one of Cassie's irrational decisions from biting everyone in the ass.
I think there's a theory going around that a YMP Yeerk was given to her, and the controller just lied to make Cassie feel better. After all it's established in book 8 that the Yeerks don't take too well to hosts running around free when something happens to the Yeerk. KA didn't want Cassie being bothered by a young girl being killed.
