"Fake" is when you don't get what you're led to believe you're getting. If I buy a poster of a Picasso, I know what I'm getting, so it's not fake. I guess I'm just a fan of science, but it would not bother me that they took video of polar bear births in a "fake" (i.e. constructed) den if I'm told, outright, that's what it is.
"Nature films" are a form of journalism, and should be held to the same standard. It's OK to stage something for educational purposes so long as it's plainly disclosed.
I do not accept at all that everything we see in image or video should be presumed "fake" (manipulated, staged, fauxtoshopped, etc) unless specifically stated otherwise. Everything has context: what looks "real" to a reasonable viewer ought to be presumed "real." Aesthetic does not reign supreme.