Cabin crew do sometimes get over-zealous... I have also been stopped from taking photographs on a BA flight. We were just south of Greenland, the seatbelt signs were off and the view was beautiful.
When I asked "why?" I was told "It's the law". Total crap of course....
On another (later) flight, the BA crew were happy for me to take photographs of London so long as I put my camera away as soon as the "cabin crew be seated for landing" announcement was made.
http://www.vampireparrot.com/gallery/LondonFromTheAir