Did you read the opinion? Williams lost.
Williams was asking the courts to force the prosecutor to present exculpatory evidence to the grand jury. Scalia ruled against him, holding that the courts don't interfere in the grand jury's proceedings except for: (1) calling the grand jury into existence, (2) swearing the grand jurors in, and (3) enforcing (or refusing to enforce) grand jury subpoenas.
Yes, the other way around. Was in a hurry. The point remains, a grand jury need not be created by a prosecutor, or convened by a court. Organizers can file with a court to take advantage of the Court’s subpoena powers, and odds are there will be a federal court that will accommodate.
A grand jury is not created to determine guilt or innocence, just evidence of wrongdoing, of probable cause. Judicial Watch could ask a Court for permission to impanel a grand jury. It need not be a prosecutor.
Yeah, Scalia would have ruled FOR Williams had he seen plainly that grand juries were constitutional only under the judicial branch.