"Very" is a word of conviction. I'm not saying I disagree, but I'm curious why you would think this...
I touched on why and EastSide described it in further detail.
Aside from the technicalities, the train of thought in that reasoning is what bothers me.
Based solely on that logic, then it should also follow that it's up to the parents not to raise criminals. Some circumstances cannot be feasibly controlled unless the parent/guardian is there next to their kid 24/7. Even then, it's not full-proof. Some people are just bastards no matter how good a family raises them.
Now that is a pretty extreme example, but it's for the sake of trying to relay my reasoning. It's the parents responsibility to make their children wear helmets then it's also their responsibility to make their children abide by the law.
I believe we humans as a whole need some sort of guidance through rules. For every 1 person who can function just fine without laws, there are probably hundreds or thousands more who wouldn't. Look at riot situations. I'm sure a lot of those people who loot during a riot normally wouldn't otherwise. They see a moment of lawlessness and go for it. I'm sure at least some of these people came from a good home.
Group mentality is hard to battle. Minors are especially susceptible to group mentality. Peer pressure. Your kid is surrounded by others not wearing a helmet. What can you possibly do as a parent to ensure they keep that helmet on their head? What makes you a good parent in this situation? If there is a mandatory law requiring your kid to wear a helmet, wouldn't you as a parent welcome that said law if you have exhausted all other resources? I guess you can lock your kid up in the house to prevent them from riding at all, but come on... this can go right back to the "Yes mom, I'll use it (lies)".
That's not such a far fetched scenario either. I've lied to my parents plenty of times growing up to do what I wanted without them knowing. I'm pretty sure I'm not alone on that.
I probably wasn't very clear with a lot of this as I'm rushing to post it haha.