"whether the person had a lawful reason to be carrying the weapon used"

But that's the point - if the courts have found that defense is lawful, then it becomes a question of why it's possession (not even use and proportionality) would not be. Then you end up in a weird state where people can make up reasons to have a hammer or something else on them rather than have something potentially more reasonable/effective like pepper spray. Allowing some limited non-lethal tool seems reasonable if defense is actually something to support.