It is good thinking my friend and you are right in saying that Itachi obtained those weapon from someone or somewhere but that does not mean that he can be disarmed by those since the trick lies in the way he uses\keeps them,
he was somehow able to fuse those weapon with his Susanoo so when he uses Susano (a manifestation of his chakra) the weapon also becomes part of it, meaning it is also becomes a manifestation of his own chakra thus rendering the possibility of being taken away by some one else useless,
since he can easily deactivate his Susanoo and make the weapon itself disappear, and then he can reactivate his Susanoo with all the previous features, meaning those weapon can not be taken away from him unless he himself releases them from the possession of his Susanoo.
as seen with Madara uchiha, Tsunade broke the sword of his Susanoo and tried to hit him with it but when he reactivated his Susanoo he had the sword's with him.