You can't fire a worker for organizing, or on the basis of sex, race, etc. But you CAN fire whoever you want for breaking even the most minor rules or even just poor performance. And the choice can be as politically motivated as google likes, doesn't change a thing.
Furthermore, google has a really easy time with "selective enforcement" They can show that people violating security rules and abusing administrative rights are always fired. Now, she can argue she didn't do this, but because of the nature of our legal system, it will be extremely hard to explain. I main, disgruntled employee abusing security? Whoever heard of such a thing. The civil judge will probably help google refer her to the FBI!
>They can show that people violating security rules and abusing administrative rights are always fired.
Yes, that agrees with what I was saying: if every message thusly inserted resulted in a firing, then it would not be fair to say she got fired "for notifying employees of their right to organize".
(I don't know enough of the history of Google's firings and this kind of violation to know either way.)