MLeake
We are discussing multiple issues here.
One issue is responsible handling and storage of firearms. I agree that in cases where there is any likelihood of unsafe persons (kids, mental incompetents, etc), the best practice is to lock up firearms. As I noted earlier, I secure my own.
Another issue is one of legal liability. While I do think the responsible thing to do is to keep one's firearms out of the hands of the unsafe, I don't think legal liability should attach for the actions of people who are not members of the household, period.
Firearms generate an emotional response, particularly from antis, and I understand that. Logically, however, if a kid wanders into my barn, grabs a pitchfork, axe, or chainsaw and then hurts another kid with it, should I expect a horde of accusers to scream at me for not locking up my pitchfork, axe, or chainsaw?
Should I expect to be held morally and legally liable if a family member's guest steals the keys to my truck from the kitchen counter, then steals my truck and has an accident?
If the answers to the above two questions were "no," then please explain the legal/moral difference with regard to firearms.
As a practical matter, it's safest to lock them up. As a legal matter, this should not be mandated, nor should the failure to lock up guns be any more criminal than the failure to lock up car keys, kitchen knives, or liquor cabinets.