Wait, but they weren't identified, right? (I don't know of the blog, so I'm just going on what bugsy says above.) How should they have the right to fire her?
Oh, well, sure, then by definition they have the right to fire her for wearing the wrong color shirt or something - and I'm not sure how that answers the question of how they're justified in firing her for exposing ... nothing at all about her students.
Now imagine it - students all knowing that it doesn't freaking matter that they don't amount to squat; they'll never have to face it.
But my point is a lot weaker than I thought it was - I had the impression (not having read the freaking original blog) that her blog was also anonymous. If it was plain who she was and where she taught, then yeah, that's a serious problem that requires disciplinary action.
There were constructive ways to say what she said. I know, she has the right, but they also have the right to fire her.