I had forgotten that all the others signed and witnessed Robb's decree. Also, wasn't Robb a King when he signed the decree? Granted not king of much, with the North lost but he was a King wasn't he?
He was a king in his own eyes and those of his followers... in the eyes of the Lannisters and Stannis and =their= followers he was a rebel, traitor, and would-be usurper.