Well, it sure tends to undercut my theory that this came from Warner Corporate. Somebody that high up in the chain is NOT going to reverse a firing decision like that; they do not want to create the impression that freelancers have leverage over executives, no matter who they are.
Now, it could be a Didio decision -- he's certainly had the "We have to kill somebody off to give this series a punch" approach before. (Remember when he wanted to kill off Nightwing at the end of Infinite Crisis?) Hell, could be Johns; he's the one who decided Barry Allen needed more Batman in his origin story.
I wouldn't be surprised if Snyder floated the idea of killing someone off and Simone said no, but judging by Snyder immediately coming to her defense on Twitter I don't think it was his intention that she should be fired if she didn't want to do it.
And it could just be a case of a new editor on the arguably-most-visible collection of books in all of comics deciding to throw his weight around. I'm reluctant to blame everything on a low-level editor without direct evidence, but I bet that's how DC's going to frame it, anyway. And whether it was his decision to boot her or not, there's no question that he completely botched the execution. You do NOT fire a creator over E-Mail -- you don't do it ANYBODY because basic human decency, but you don't do it to someone who's popular and selling a lot of books because of tactical reasons.
Purely hypothetical but maybe it went something like this:
Didio: Hey, this Death of the Family thing's selling really well. Who you going to kill off?
Snyder: I was thinking [some character in Batgirl. Barbara's roommate or somebody.]
Didio: Hey, tell Gail we're killing off [whoever].
Group Editor: Gail, we're killing off [whoever].
Gail: No.
Group Editor: Then you're fired.
Internet:
Didio: Oh no, not again.
We'll probably never know the details, because Gail doesn't generally talk out of school. All's well that ends well, I suppose.