I think its pretty clear that she refused to testify out of principle. None of the war criminals exposed by Manning's leaks have been charged or tried for their crimes. The government's refusal to charge the war crimes, which are central to the entire case, completely undermine any pretense of "justice". As usual, the government is using its infinite power to punish those who exposed wrongdoing, while completely ignoring the actually wrongdoing that was exposed. I find it quite admirable that Manning refuses to be used as a tool to further this charade of injustice, at great personal expense. Being held in "contempt of court" is no vice when the court, and the prosecutors, have done everything possible to earn that contempt.

For those unfamiliar with the still unprosecuted war crimes exposed by Manning (and published by Assange):

https://www.youtube.com/watch?v=5rXPrfnU3G0