The thing is, the subject of a photo, the scene and its subjects, are usually not the artists. The photographer is the artist. Photography is processing from the get-go: how the film or CCD responds to light and so on. The grain from low-light on sensitive film can be part of the art and so on.
If you mess with the colors of a scene, you're not taking away artistic control from that scene.
You also don't put limitations on the post processing art; you're not doing it to fool anyone.
There is post processing in music that is obvious art in an analogous way, like taking sampled sounds and re-mixing them to create new stuff. There are effects that are obviously effects. I'm not going to scoff at a great studio reverb, or some echo applied to a vocal or whatever. Nobody is saying that this was recorded in some fjord in Sweden with real echo bouncing off a distant ice wall; there is no lie.