How does this not make sense? Why even put shields in the game if everyone can block with a sword? In Oblivion you could just take a dagger as a mage and block without any disadvantages.
On the other hand, I think something like a parrying dagger would be a neat addition. Basically using a dagger as a shield (and not for stabbing).
But it really doesn't make sense, for certain weapons. It makes sense that you cannot hold and block, as you could with a shield. But you should at least be able to parry.
If someone swings a sword at you, and you fend off the blow with your own sword or mace, then the sword isn't going to cut into you like it would if you had tried to block with your arm. Sometimes game "balance" breaks common sense.