That's like saying he should be able to metal-bend...the Avatar doesn't necessarily have each element to the ultimate degree. Only a few select fire-benders can lightning-bend, just like to my knowledge only Toph can metal-bend.
Redirecting lightning is another story because it's just applying a water-bending philosophy to fire-bending.
Bah whatever to all that, I just hope the movie is just as if not even more epic. And who knows, they might give Iroh more ownage-time
