Wouldn't YOU want to be able to query the actions of that Unit Commander? Wouldn't YOU want the death of your son to be the subject of a trial in a public court where individuals can be held to account for their actions and their consequences?
THAT is why the battlefield should never be beyond the court room.
pr00ne. No. No. And I disagree.
As has already been pointed out, your hypothetical example is highly unlikely to happen, but if it did I would hope that the platoon commander had good reason for his decision, supported and backed up by his NCOs. A court of law, staffed by a bunch of people who have no concept of what it was like or what led to that decision are not the people to judge whether something was lawful or not; they cannot and will never understand. It's war; s**t happens. For a commander to constantly have to second guess what some ambulance chasing lawyer will create in court after the event is wrong. It will reduce commanders' flexibility and it will lead to more risk aversion; something that is already becoming prevalent in the Armed Forces. Sadly if you don't take risks we become ineffective; if you become ineffective you will lose.
That is why the battlefield should be beyond the courtroom (and it's not and arguably never has been, because using your example, there would be a military enquiry anyway).