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Both have their work looked over by an editor. And, editors are supposed to police what is published to ensure that the 'facts' are true and corroborated.
In addition, I point you to a passage from the ASNE Statement of Principles (emphasis mine):
Good faith with the reader is the foundation of good journalism. Every effort must be made to assure that the news content is accurate, free from bias and in context, and that all sides are presented fairly. Editorials, analytical articles and commentary should be held to the same standards of accuracy with respect to facts as news reports.
Sig is link but here is the URL: http://www.asne.org/kiosk/archive/principl.htm
In light of the above, I would say that opinionists and reporters CAN be held to the same standard when it comes to presenting something as fact. And, the best way to corroborate something is fact is to cite multiple sources that are not anonymous.