Hands down the only reason for this is to make the prospect of hybrid elemental/enhancement shaman very unpopular. In the patch notes it was stated that many Main Hand only weapons would be changed to one-handed which means they could be wielded in either the main or the off-hand. I would venture to say this was for the blacksmithing BOP epic weapons that are now no longer unique and can be dual wielded (I wish I never dropped blacksmithing for leatherworking) as well to give shaman, warriors, and rogues a little more options in what gear they can use. I reckone as well this is a change made for Wrath as well.
Caster weapons were not changed intentionally. (src) Hortus states plainly that caster weapons were not changed on purpose, they were not meant to be dual wielded.
Which burns my butt.
I really wanted to try a hybrid build like that — as you’re sure to recall from our first conversations here.
So it is just another case of Blizzard dumping on the shaman, keepin’ us down, yo. ^_^
LOL The Blue man is keeping us down yo.
Nah it was a nice idea while it lasted to screw around and be different. But I guess Blizzard had a really specific “look” they wanted shaman to have, either healing and shooting lightening out of your eyes with a staff or shield, or a melee naturalistic brawler.
Represent!
On a more serious note, it really is an irritant to me.
Am I seeing things or does it really seem like Blizz hates the shaman? 
I reserve all rights to QQ and start an uprising until AFTER our Wrath talents and spells are released.
Because back before Burning Crusade, when shaman were told they can dual wield and earthshield and lightening overload and wrath of air, people were crying nerf until the forums broke.
Let us hope that they are not a bunch of mindless jerks who will be the first against the wall when the revolution comes!
However, should you need a shocktrooper (Lia is elemental specced — so it would be literal!!!!) sign me up for the crusade. ^_^