Because people are totally worried about all the times that they lose claim when they intentionally hit /attack off. The fact that they used the disengage mechanic to trigger the forfeiture of a claim has absolutely nothing to do with the "ridiculousness" of sheathing your weapon.
Edit: You can use ranged abilities and spells without being engaged. You cannot use Any WS without being in Battle Mode. That does not mean there is a "glitch" with the claim system. That means that WS are not programmed to be usable outside of Battle Mode. Period.