It's just one of those "WTF?" rules that you occasionally find in the game. The Druid weapon restrictions just don't make any sense since there are too many inconsistencies. They can use scimitars but not falchions or kukri's, they can't use bows or arrows because of the metal in the arrowheads but they can use a scimitar or spear, they can use a sling but they can't throw rocks, etc.
Personally, I just ignore the restrictions if the player can give me a good reason why their Druid should be able to wield a given weapon.