Shooter was duly notified of a shot angle that was not acceptable to the range or MD, and took it anyway. A DQ is not inappropriate.

That being said, there is plenty of fail to go around. The stage design is faulty, and everyone relied upon human beings following instructions during a gun race to mitigate it. It also sounds like a range layout issue if there's an accessible, downrange-ish area protected only by a non-ballistic wall.