Charlie, if the comments on the Reason website are the entirety of the alleged threat, then I agree with you that the subpoena is abusive. But consider another possibility: What if real but anonymous threats have been made against the judge (perhaps, for example, in letters mailed to her residence or chambers) and the website comments bear a striking similarity to the real threats? Then the subpoena might well be a reasonable part of a criminal investigation.
On a lighter note, I was struck by the infelicitous phrasing in the letter accompanying the subpoena: “Please be advised that the accompanying grand jury subpoena has been issued in connection with an official criminal investigation of a suspected felony being conducted by a federal grand jury.”