She was a senior exec. You don't keep those people around when they're going off to work for a rival.
I admit I haven't been following the drama that Mayer has been creating in her wake, but if she really quit Google with only a 30 minute notice, this strongly indicates that she really didn't like the place to begin with. If she did, she wouldn't do anything to burn bridges so thoroughly.
I wonder what she pays her PR staff. Every time she has a bowel movement, it seems to show up as a businessinsider.com exclusive.