While it is unfortunately not uncommon for mistakes and errors to occur in military training exercises, the United States military conducts these exercises with their soldiers in mind. After all, the exercises mean nothing if the soldiers executing them are seriously harmed. The unfortunate part is the stakes are often high with the exercises, meaning when something does go wrong it can be fatal.
The US works hard to ensure the safety of its service members during training. But the military is not intended to be a danger-free profession. Like police officers and fire fighters, service members willingly take on a degree of risk. There will always be some level of risk involved in training, due to the inexperience of the trainees.
Serving in the U.S. Military is a dangerous business. In addition to regular combat, part of the danger involves training. The reality is that it is expensive to train and maintain soldiers. It serves no purpose for the military to not take adequate precautions to protect its primary assets - its people.
It is of course extremely unfortunate when someone dies during military training; however, war is not exactly a safe place, either. It is important for soldiers to understand their own mortality, and a bit of danger in training can emphasize that. I do not wish for anything poor to happen to those in training, but it is a potential consequence they agreed to.