Yeah, my confusion. At one point there was a FAQ allowing you to kill the rider. It came up because you can actually target the rider independently from the mount in combat, and it made no sense at all that his neck suddenly got thicker just because he was riding something. GW FAQ'd it with reason and allowed the KB to work. Later on they changed it.
Now then, here's the problem they've created - It's not out of the realm of possibility claim that a single HEROIC KB can kill both the rider and his mount at once. It's a stretch, but still...