"What makes it a police concern is that doing otherwise is inviting auto theft, which is a police matter. "
hmmm Leaving the garage door open is similarly inviting property theft. Our Police State masters must write a revenue-generating ticket. It's for our own good, ya know.
Hey wait, buddy...the gate to your back yard is wide open. Anyone can just walk in...
Stop, Reggie - back up! I see a car on the street with the windows down...
Unlike the opposing example you present as equivalent, auto theft puts others at risk in several ways. Kids could drive away with the cars and get into accidents. Thieves who steal a car can use that car to flee when cops catch up with them and thus result in a dangerous high-speed, light-running car chase. Etc....
Someone would have to be dumb enough to have unsecured weapons stored in an open garage to create an equivalent risk. I would not waste my time sympathizing for such a fool.