I totally thought the same thing. XD
My interpretation of it was always that Nightblood, when sheathed, was in a functionally "dormant" state and thus didn't require Breaths. When drawn, however, that's when a weapon becomes "active" and, in this case, is when Nightblood would be consuming Breaths in order to do things. After all, a sheathed sword isn't good for much, except as a bludgeoning weapon.
Besides, anyone smart enough to create Nightblood in the first place would probably be smart enough to make it so that he's not constantly drawing Breaths every second of every day.