Resigning and talking about it is actually pretty courageous. She doesn't win, she gets a stigma. Her company doesn't win, it gets a stigma. But the spectators with their mouths agog realize that this kind of mismanagement/harassment is not only happening under noses, but also being noticed externally (it's endemic; while women and minorities likely deal with it a lot more, it's not just gender-based, it often happens to the lawful-good no matter what gender or race in small tech companies) and, for a brief time, these companies pull themselves together and become better places to work, if only because it's expensive and time-consuming and morale-killing to fuck this stuff up.
If you had read the original article [1] you'd have known that she decided to take action after a "Secret post" was publicly published about her departure. (We could comment all day about the bravery of the anonymous GitHub employees who used Secret as a public mean of ridiculing her)
[1] http://techcrunch.com/2014/03/15/julie-ann-horvath-describes...
It is also easier for her to go public than to try to sue a billion dollar company, and have to be involved in a lawsuit for years. Nobody wants that hassle.
Bullies are able to operate because people keep quiet. Going public is often the best way to defeat a bully. The founder, his wife, and the employee who ripped out her code never thought the general public would know about the things they did. If they did, they would never have done them.
Which would have put Github instantly on the defensive, and would have lead to limited and speculative publicity only. As it is, Github is not being sued, and seems to be making a proper effort to address its problems. This sounds better, really.
Not necessarily. It can put pressure on the company to take it seriously and fix it, and make more people aware of these kind of problems. And that seems to be what's happening here. Keeping it under wraps allows the situation to continue as it was.