The subpoena cites a law against "interstate threats" as the reason for demanding the information, which the Supreme Court very recently decided must include real intent.
As White points out, the comments — repugnant as they are — may very well not constitute a true threat, as they aren't directed at the judge and don't detail any real plans for violence. The kicker: although it's possible to fight the subpoena, precedent suggests the US Attorney's office may have the power to obtain the information anyway.
However the situation shakes out, this isn't nearly the first fight over commenter anonymity and the First Amendment, and certainly won't be the last.
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