I know, I say in theory it shouldn't be possible, because that wasn't in the original specs, but in practice it was easier this way, and I figured that if you can heal a serious wound a scratch shouldn't be a problem, so I left it in as an 'undocumented feature' of sorts
With that commit I was thinking more of someone clicking the wrong soldier, or trying to heal a stunned alien for capture, or just failing to realize the soldier was already healed, wasting 20TU and one of the precious few medikit charges seemed a little harsh for those situations