Things are what they are, not what our arbitrary categories classify them. A knife is a tool and a weapon at the same time, and it can be a great number of other things, a cultural artifact, a piece of industrial design and a piece of junk, all at the same time. A thermonuclear warhead device can be a doomsday device and an essential tool for Mars terraformation.

So it's not the way you use it that makes a weapon, a weapon remains a weapon throughout it's non-violent uses; the oldest trick in war is to disguise weapons as tools before using them in an attack. See nuclear proliferation under the guise of atomic energy, the militarization of space, Hitler's powerful 'defensive' army.

It may sound like a pedantic distinction, but in practice, the fact that something IS a weapon, regardless of how it's being used, will drastically alter decisions. Imagine an unknown person coming at your family with a knife or ax in hand. Under the "not a weapon until used as such" doctrine, the burden is on you to prove the agresive intentions and you have no logical reason to be concerned until then.