Apologies for not having my JPM (our Joint Procedures Manual which details all of our safety procedures), but there is a clause somewhere in there about photography only being allowed with the permission of the Captain. The crew are then expected to report to the Captain any activity which might compromise safety or security.
The vast majority of us will always encourage people to make memories and gladly take photos of families or couples onboard the aircraft. Often customers in First or Club also like to photograph the seat or the food. I think crew are just very aware that the comfort and well being of other customers and of the crew can be compromised by some people's photography. I'm more than happy to pose for a photo with someone, so long as my name badge has been obscured and I've had a chance to fix my hair. I have however had people shove a camera on my face as they've boarded and similarly had to collar someone for trying to take a photo up a colleague's skirt. I think sometimes these bad eggs just puts us all a bit on edge, ruining it for the genuine ones.