The only reason we get this discussion about photographs and cheating is that we remember the important photos we've seen of the American Civil War, the civil rights struggle, the demonstrations in Egypt, etc. that move us because they are "real". Paintings and drawings of the same events don't normally move us in the same way because they are not "real". We then assume that all photos are real in a way that other art is not. We see a portrait photo and assume that the person really looks like that--there might be a few blotches retouched out, but by and large we expect that the photo is what the person really looks like. But we don't hold the same belief for a painted portrait--we (or at least I) assume that the painter took pains to make the subject look as attractive as possible and did more than retouch a few blotches. This expectation that a photograph represents reality is just something we have to face. It's OK if we retouch or manipulate to better represent our vision, but we have to expect a few to object--how many object often depends on what our vision or intent is.
Defining cheating is not easy and depends on the photographer's intent and our conventions and expectations. I don't think anyone really expects advertising and clamor photographs to represent reality and we accept a great deal of manipulation in these. Journalism photographs we expect to be real and un-manipulated.
For example: Assume that I'm out hiking and come across a beautiful small stream and think what a great picture this will make.
As I'm setting up, I notice that someone has left a dozen beer cans, several fast food bags, and an old tire that pretty much destroy the beauty. If I'm taking photos for the chamber of commerce to show of the beauty of the area I can:
1. Change my viewpoint by moving slightly and changing the lens and the mess disappears and I've got my beautiful picture--is this a cheat?
2. Take the original photo and clone out the mess--is this a cheat?
3. Remove the mess, take the photo, and carry the mess to the dump--is this a cheat?
Or assume that I want to show what slobs are doing to destroy the beauty of the countryside and I change my viewpoint by moving and changing lens to emphasize the mess--is this a cheat? If I manipulate the photo to add more emphasis to the mess, is that cheating?