That doesn't appear to be entirely correct. There is an investigation of a possible crime and her testimony has been sought. We don't KNOW that any crime is involved. I doubt that there was outside of maybe conspiracy to undermine National Security.
Miller is in jail for Contempt of Court.
Yes, Miller is in jail for contempt of court. Why is she in contempt? Because she refuses to testify.
Is leaking classified information still a crime? If so, was she a witness to that crime being committed?
You have to testify about a crime if you are a material witness; it is not optional...not for *any* crime.