Why it's a bad idea to over-edit

There is a reason editors exist, and I think that reason is simply the need to slap writers' hands and stop them from editing where no editing is needed.

So I'm proofreading The Badlings, right? And this thing has already been through 5 drafts and 2 editing scrubs and is supposed to be final, then I catch this clumsydescription that is very important. It shows a major world change, yet I glossed over it. Here it is (WARNING!!!Spoilers.):

"A faint ahhh went up from them, and for an instant they...