Earlier quoted context omitted.
She's not locked up for leaking, she's locked up for flagrant contempt in an active grand jury investigation, one which otherwise poses her no legal jeopardy, since she has both immunity and, for much of the subject matter, double jeopardy protection since she's been tried, convicted, and served her sentence for her involvement.
Principles aren’t principles if you only have them when they’re easy. The US government shoved her in isolation for years. Even with immunity why should she help them now?
Because if her previous testimony was not perjury, and the government is looking to probe around it, she is likely withholding exculpatory evidence that, were it in the hands of the government, they would be subject to sanction if they failed to turn over to the ultimate target of the prosecution and which might, simply by existing, result in abandonment of some or all of the charges being pursued.
Of course, if the prior testimony was perjury, the calculus is different.