Old Carthusian;
If the crew had followed SOPs, CRM and the procedure for UAS this accident wouldn't have happened.
Perhaps the human factors group which the BEA's Alain Bouillard has implemented and directed to investigate these questions will learn why this was so. I wonder if there was a "cockpit gradient" issue - two F/O's do not have the same nature of authority and decision-making that a Captain/FO crew do. The discussion, which I know has been had here, about who was in command "really", as well as actually, is only part of this notion. In fact I get the subtle sense that the gradient was from right to left - higher in the right seat even though he was the junior crew member.
IIRC, clear direction was not provided by the captain on which crew member was to command the airplane, vice who was just going to fly.
Making a firm decision in such circumstances can be problematic depending upon company policies, training and even personalities.
Some airlines do not utilize two First Officers in Long Haul operations and provide two separate crews - Captain and F/O, to relieve the first crew. Whether this will turn up as an issue or even a comment we cannot say depending upon other factors that the Human Factors Group will discover.