I think that is has to do with the "stability" of the avod system used. For example, I have been frequent user of TG. Their "old" system on 747 do not allow it. But the 77W (from 9W0 used on the CDG run is different and the system is on when you walk on the plane. Same on their new A330s used regionally. Sorry, but I have not time to find the relevant "technical" threads, but there was a discussion when BA introduced the new system. See also:
http://www.terminalu.com/air-travel-...landing/31594/

Of course, one still has to attach the screen for takeoff, but you can use it immediately after takeoff even f the seat belt sign is on and FAs remain seated. Even if you cannot watch the movie, you still have a wide music selection as the system itself is not disconnected. In any case, I do not think that the captain bothers with a decision to start the system. I believe that the purser does it when they are authorized to stand up. Launching and initializing a computer system for each cabin takes a long time. So it typically takes 20 min or more after takeoff. When boarding BA or TG planes, the system was already active.