This is a fantasy game, nuff said.
It is pretty lame, i can understand the AoE argument, but in that case like REL said you would just have learn from that mistake, just like you would if you jumped off a cliff on your horse... What do you thinks gonna happen?
Getting pretty tierd of people bringing realism into this... This is a fantasy rpg game with fantastical creatures, and among all things magic, this game is about as real as LOTR is.
Honestly, my theory is that they had alot of codes and scripts to deal with and program to get magic working on Horses, it could have created more problems/bugs in the end, and therefore they did not want to include it in the initial release (leaning more towards it being too time consuming), therefore they excluded magic so they could rush mounted combat into the game before Dawnguard came out, which they've succeeded by the looks of it.
Dual-casted spells can only be cast while stationary, so I highly doubt it would even be possible to use while on horse. A moving firestorm would be kind of silly, really...And probably pretty hard to code.
In any case, they will hopefully add it in at a later time. Maybe in the next expansion they will add tons of new magic features and add magic while on horse as one of those features to market their new DLC?