If someone cuts the brake lines on your car and it takes off down a hill and kills someone because you can't stop, do you go to prison? After all, you're the driver, ultimate responsibility must lie with you and you should have crawled under the car to check the condition of your brakes and make sure nobody cut them.
Is crawling under your car before driving every day the normal course of due caution? I have never met a person that held the belief that inspection of the brake lines before each use was a common and prudent act.
But, the belief that a weapon be checked and handled with due caution is a widespread belief. The 4 rules are not an obscure belief but are quite commonly known and adhered to.