This is a tough rule for some writers to get their heads around. Before I weigh in on it, you need to read two very excellent posts on the topic, both on Dean Wesley Smith's blog: here and then here. Go read them, I'll wait.
Okay, everyone back? Let's chat about this.
I don't know why people get angry about this. But apparently they do. Can someone explain it to me?
Maybe it doesn't bother me because I was blissfully ignorant of this myth for the most part. I get the reasoning as well. Writing and rewriting require two different knds of thought processes, and the two of them don't get along. If you go back over your work with your critical brain running the show, you will probably kill all the special parts of your story.
Remember my tale about the story I wrote that I thought was garbage but everyone else thought was great? If I had gone back into the story with critical voice, I would've changed all kinds of things. And I'm willing to bet it wouldn't have been nearly as good as it was.
But does that mean all first drafts are great stories? Not at all. The best way to understand it is to think of a manuscript as a tool. You are using that tool to tell your story that's locked in your brain.
Sometimes the manuscript you write is the wrong kind of tool for the job. Like using a screwdriver when a hammer is called for. There's no point in trying to change the screwdriver into a hammer - the right thing to do is to find a hammer.
So if you write a bad manuscript the first time around? Well, you take the idea and go for the correct tool. In other words you redraft the story, writing it again from the beginning. And when you do that you're still working in creative mode, which is the mindset you need to be in when you're writing your fiction.
But why the editorial exception? Couple of reasons. For one thing, an editor can give you money for your work, so it pays (pardon the pun) to listen to them. And an editor knows how to tweak a story to make it sell.
That doesn't mean you have to do all they tell you. If you disagree with something an editor says, you should think twice about making the change. But I've found often I'll see an editor's suggestion and think, "hmm...good idea there." And it makes the story better.
Again, I'd really like to know why some people freak out over this rule. Maybe a reader can clue me in?