Huh, I took my own passport photo, adjusted the colors, fixed some red eye and even touched up some blemishes and dark circles under my eyes. Why make a rule if you have absolutely zero intent or ability to enforce it?
I did the same thing, and it's obvious they know people are going to do it. So why make the rule? I suspect it is so that you can't argue about whether it is or isn't touched up too much if they don't happen to like it. If they can show any touching up at all, they can drop the hammer on you.
