It was an ACCIDENT ! You arm your soldiers and THEY should be smart enough to put on their OWN seatbelts.
Sometimes, like this, when a tragic ACCIDENT takes place, we look to blame someone. I am sorry to hear a life was lost, but isn't it up to each of US when it comes to our safety? Buckling a seatbelt should be common sense. If the troops were NOT following orders and goofing off, that is one thing, but if this accident occurred during a normal training session, then it is clearly an accident.
Do you think the brass wanted one of their troops to pass? Absolutely not, so train them to enforce the safety rules, don't discipline them though.