Actually I never knew this. But when I went back and check the rules in the BRB, low and behold, they do infact say that when a character is on a monster the entire model counts it's troop type as "monster" (infact it says you change the characters unit type for all mounts, so no killing blow a character on a chariot either). So the FAQ actually didn't change anything, the rule has always been in the book! Learn something new every day.
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...