As for my speculation, given the rejection statistics, I'm guessing the HS facilitators could have chosen to admit many more females before this would have even been an issue that should have borne consideration.
"We're not going to lower the bar for female applicants. It frustrates us a little that we feel the need to say that, and we think it underlines the sexism (intentional and not) that so pervades the programming world.
But we want to say that now, so people don't have to waste time asking or debating the point. Women will be judged on the exact same scale as men. We think to do otherwise would be insulting and counterproductive. We care a lot about getting more women into Hacker School, but we won't do it at the expense of the quality of the batch."
From: https://www.hackerschool.com/blog/1-summer-2012-applications...
Now. On a purely practical note, HS is a business, and the core objective is to deliver excellent batch dynamics. The responsibility is to attendees and attendees only -- how are they delivering an experience that makes the time (and, indirectly, money) commitment of the students worth it? The issue of "fairness" does not enter the conversation because this is not a charity. Or, more straightforwardly, their responsibility is not to people who did not get in.
Now, ok, you might argue that you are not convinced that having a roughly-equal gender ratio is beneficial to the batch as a whole, but I am in the current batch and I'm telling you it makes a huge difference. If you don't believe me, let's chat over email or skype.