It's the idea that you shouldn't be meddling with nature, which is a separate idea as to whether a photograph is realistic.
Let's look at it this way, if I shoot a wolf in beautiful light feasting on a carcass, it's a nice photograph but the light isn't going to be wonderful all the time and the woods are nowhere as beautiful as that single primitive moment. That relates to the idea that photographs are not exactly representative of reality (you can of course take this further as you wish).
That's quite different from baiting animals to come out into the light, which they might not have done without the bait. So for example if I lay 10 beautiful succulent fish on a rock bathed in sunlight to entice a kingfisher to stay perched so that I can shoot more conveniently, that's meddling with the animal's behaviour. Do fish normally bask in sunlight on rocks for the kingfisher? Nope, the kingfisher hunts it down and eats it. Some may even argue that prolonged exposure to this sort of thing leads to expectations that there will always be fish on the rock. That could lead to all sorts of warped consequences like the kingfisher being exposed to predators, risks that it would not normally take, long after the photographer has gotten his prize winning shot and moved on.
There are certain genres of photography which have written or unwritten codes. Photojournalism is one, and I would think nature/wildlife photography is another, especially when animals are involved. Portrayal of unrealistic behaviour such as frogs sitting upright, lizards dancing (read:
http://petapixel.com/2013/08/20/pho...s-of-faking-cute-animal-images-in-cruel-ways/ ) are not only perceived as a betrayal to the excitement of the hunt of tracking down the animal and capturing it in the right moment, but also involve unnatural/cruel manipulation of the subject (how would you like it if I strung you up in a contorted position that you would not otherwise adopt?). There's also extensions of what the photograph may mean - if a frog sits upright and this is observed that has all sorts of scientific implications. If you move a soldier around or manipulate the resultant photograph to portray something different from reality political statements are made.
This is different from the arguments for and against composite photography that is linked in the article UncleFai has posted. That's totally different and for other genres such as fashion, conceptual, fine art, all sorts of manipulation is used. For example, I don't suppose anyone really believes that food materializes as it does in Carl Warner's beautiful
foodscapes.
