You've missed a step, the step where the Afghani checks his car and doesn't take the rocket launcher to school by mistake. The toy gun is perfectly permissible at the kid's home.
A better war time example would be someone who has a live IED in their carry-on for the flight home. "I didn't know it was in my bag, I thought I left it at the base."
If that soldier unintentionally put the lives of everyone on the airplane at risk, does she still get a pass? An airplane, like a school, is a controlled environment with very strict rules. Should it matter if you didn't know you had a gun in your briefcase, or should the pilot kick you off the plane when you turn it in regardless?
The benefit of a strict policy is that it avoids having to determine whether the "I didn't know" is a lie or not.
I'm smuggling cocaine. I get cold feet. I show the drugs to the authorities and claim I didn't know. Am I still guilty of possession? Should I have known?
This whole idea of dodging responsibility by citing ignorance is offensive.
May I suggest reading through this article:
http://en.wikipedia.org/wiki/Mens_rea
First of all, this thread is no longer about this boy and his toy gun specifically, but about ZTP and "strict liability". They are related, and I detest both of them (except for minor traffic violations, then "strict liability" can be appropriate).
Ignoring mens rea in favor of "strict liability" is IMO both unnecessary and leads to unintended consequences.
Your soldier returning home with an IED in his possession doesn't require strict liability. This is because he acted "negligently". Is it reasonable that a solider knows that he doesn't have any explosive devises before boarding an airplane? Yes. Is it reasonable for a 1st grader to check his backpack each and every morning before going to school in case his mom left something in there he can't take to school? No.
Your drugs smuggling example: No, you shouldn't just be let go. If in fact you knew you were smuggling drugs. Given a lenient sentence for turning them and giving testimony against whomever sold them to you, sure. But, you are ignoring the possibility that someone, at some point in history, has had drugs, weapons, or explosions planted in their vehicle or baggage without their knowledge. Should that possibility just be ignored and anyone who turns in a k of coke spend the next 10 years in prison, even if their story is plausible? No? Well you don't really support strict liability.
What if I am shopping at the grocery store and when walking out to my car I realize, they forget to charge me for that case of beer at the bottom of the cart. I'd go back and pay for it. But, under strict liability I shoplifted. If I thought there was a real chance that instead of thanking me for my honesty, the manager would call the police and have me arrested, forget about it. The store just lost out on some cash they could have had. Maybe you and a few other "Dudley-do-rights" would turn it in and face the consequences, the vast majority of people wouldn't.