It's quite true that any potential character can wield any of these "only for NPC" weapons. The major stumbling block for Yoshimo's Katana, for instanace, is that you have to be true neutral. You also have to be either Human or Half-Orc. Other than that you can be lots of classes that could potentially wield any Katana. I used Shadow Keeper to make my fighter true neutral and now he can wield Yoshimo's Katana. Wow, I have to say my respect for the implementation of this game has been lowered somewhat. If the Devs considered it important to the storyline to have items useable only by certain NPCs, which is implied by the item's description, then they should have made dang sure that it was the case. Otherwise...why bother? I don't get it.
On the other hand I'm glad I know this. I mean, since it's not really possible to make a weapon NPC specific then I'm not going to bother trying to make my weapon PC specific even if I could do it with scripting. The whole concept shouldn't even be in the game's item descriptions to begin with if you ask me.