That would be extra wonky, really. Plus, even when you block an attack with a shield, it can still inflict blunt trauma to the offending arm or get through the shield depending on the nature of the attack.
As it stands, making it armor seems a little better than having it randomly outright negate attacks. The catch is, in addition to being unable to use it offensively, the “you can still be hurt through the shield” mechanic only represents the targeted body part getting hit, there’s no mechanic for getting your arm broken from blocking a blow that was supposed to bash your head in. XP