The caster can have this weapon in their inventory. They just can't have it in their hands while casting a spell. This way, a caster who also wants to use a sword and shield during combat must store one of them on their body while casting a spell. It is mostly a role play thing, but it can cause some trouble when a caster wants to cast a spell during combat and has to redraw their weapon afterward before they can attack with it again (something which might cost them a round to do)