In defence of the shot of the polar bear birth - The mother would have chosen that spot, dug herself a cave and settled herself into it months before the birth, and not left it until well after they had arrived. It is not a den which she leaves regularly to forage for food, so that a camera could be conveniently installed while she was away. She would be there all the time, and would not appreciate being disturbed.
If the BEEB team had tried to install a camera while she was lying there, then, as David Attenburgh said on ITV1's This Morning: "If you had tried to put a camera in the wild in a polar bear den, she would either have killed the cub or she would have killed the cameraman, one or the other."
"To explain mid-programme that this one sequence was not from the wild would have upset the programme's atmosphere, he added: "It's not falsehood and we don't keep it secret either."
There was a separate film on the Frozen Planet website explaining the background to the polar bear cub footage, which shows the hidden cameras being fitted at the wildlife centre weeks before the polar bear gave birth.
All documentaries are edited and voice-overs and explanatory inserts added. (Apparently among the tweets about the captive filming, there were also some mock-outraged tweets. One read: "I'm outraged to find out the soundtrack for #FrozenPlanet was added after filming. I thought they took a full orchestra with them.")