the only capital crime we have is treason in time of war. This is in place for use on the battlefield.

After all, you never know when you might need to be able to shoot for desertion etc. It gives the commanders in the field the room that they absolutely have to have to maintain discipline without worrying about charges.

This assumes that some other soldier doesn't disagree with the commander. This makes using this very rare, but means it can be there when absolutely necessary.