John, I completely disagree. Surprising, right? 
Where is the moral imperative to disclose what was done in an artistic image? Art is an entirely subjective arena. Gursky created the scene as he saw it, or wanted to see it. The goal, with documentary/PJ is to try and maintain objectivity.
I'd suggest a read of this essay by Guy Tal, http://guytal.com/wordpress/2011/02/12/lie-like-you-mean-it/.
Okay, I read the essay, and you know, it doesn't do much for me, because it doesn't reach any kind of defensible conclusion. Like you, I think all kinds of artifice is allowable in art. (I'm more of a painter than a photographer, and though my images look somewhat realistic, they obviously aren't an image of any scene that anybody's ever experienced.) But photography presents a peculiar and singular problem: that is, since its inception, photos have been presented as images of a certain kind of objective reality. It hasn't always been that, and there have been manipulations from the beginning, but of the billions of photos taken since the beginning, probably 99 percent are unmanipulated, straight-out-of-the-camera family photos, cat photos, snapshots, etc. Few iPhone photos are post-processed, because that's not what iPhone photos are for. And it's true, as Tal said, that there may be a roaring expressway a few inches out of the landscape photo, but if the photo itself is unmanipulated, then its an image of what was in front of the camera. So: the default position of a photo is that it's unmanipulated. It always has been that, and in general, manipulations before Photoshop were self-disclosing.
Art photos may either be manipulated or unmanipulated. Either is equally acceptable.
However, if you present for sale a landscape photo that gives every appearance of being unmanipulated, which is the default state of out-of-camera photographs, and in fact you manipulated, but then essentially deny the manipulation, you're being unethical. You're lying. Painters essentially can't lie about the images in their works: the images are what they are, individual creations that spring from the minds of the artists. But photographers, because of the peculiar mechanical method of making a photo, and because of photography's history as an "objective" medium, can lie, and they do, and that behavior is unethical. It has nothing to do with whether a piece is art or not, or whether its been manipulated or not, it's whether the photographer is lying.
To make it brutally clear: If you are in the American southwest, and you take a photo of an arch, and then you uses Photoshop to bring in a separate moonrise, and place the moon in the arch, I have no problem, though I wouldn't buy the photo -- it'd just be another inane Photoshop idea. But if somebody asks, "Does this represent what actually happened out there?" and you say "Yes," then I do have a problem. If you represent the photograph as an image of a natural fact, you're being unethical, no matter how beautiful the object (the photograph) may be.
