That's a matter of opinion, but I think its like this. The initial target is just an epicentre of a "blast radius" of fear, like how volley fire used to work. Sorcerous blast only literally affects one guy - it chucks him about. If he hits someone else? That's physics, not magic. He's been punched, and moves a bit, and hits someone else who's probably not expecting it. The Panic steed is simultaneously causing all the horses/wargs/dragonypentrodayctlthingies to feel fear and want to run away. Only by mastering their steed can a hero inspire the horse/etc to stick around.
If a spear model was transfixed, the guy he was going to support can't do a thing about it - he is an indirect victim. An uruk-hai captain that's just had an unlucky subordinate in full armour propelled at him is also an indirect victim, just in a more immediately lethal way. But a bunch of horses wanting to run away at the same time are direct victims.
If you want to play it your way that's fine though.
I just feel that that way doesn't add a lot to a game (could have some sneaky exploit, whatever). But all IMHO.