As a civilian, I have the expectation of safety in a typical public space and will deal with random incident as it happens.
If a tree happens to fall on me or a family member as we're walking down the street, so be it. In that unlikely circumstance, we will not have time to ponder the wisdom of wearing a thick helmet when there are trees around. We will be too busy dead or dying.
I'm not going to walk around wearing a thick helmet to cushion my skull from random treefalls, and I'm NOT going to walk around carrying a gun to shoot dead some lunatic randomly shooting their gun at me.