When does a massacre matter?

I ask this question, because on Thursday, June 28, The Associated Press—and to a lesser extent, Reuters, and a small independent Iraqi news agency—ran stories claiming that 20 decapitated bodies had been found on or near the banks of the Tigris River in Um al-Abeed, a village near Salman Pak, southeast of Baghdad, with sectarian violence strongly implicated. . . .

This claimed massacre never happened, and was formally repudiated by the U.S. military on Saturday, June 30, who ascribed the claims to insurgent propaganda. To date, the Associated Press has refused to print a retraction or a correction for this false story, just as it has failed to print a retraction for previous false beheading stories. . . .

At the same time, the Associated Press has refused to run the story of a verified massacre in Iraq discovered on June 29 and supported by named sources, eyewitness statements, and photographic evidence provided by noted independent journalist Michael Yon in his dispatch, Bless the Beasts and Children.

I would like for the Associated Press to formally explain why they are willing to run thinly and falsely sourced insurgent propaganda as unquestioned fact without any independent verification, but refuses to publish a freely offered account by a noted combat corespondent that some consider this generation's Ernie Pyle.

Is it because the massacre documented by Yon was conducted by alleged al Qaeda in Iraq terrorists, and could not be ascribed to sectarian violence? It certainly could not be because of cost, as Yon has offered both his text and pictures to any and all media outlets free of charge. It could not be because of a question of validity, as his account was photographed, videotaped, and witnessed by dozens of American and Iraqi soldiers, some of them named, who could easily be contacted by the Associated Press for independent, on the record confirmation.

Why is the Associated Press willing to run the claimed of a false massacre on June 28, but unwilling to report a well-documented and freely-offered account of a massacre that was discovered just one day later?

I await your response with interest.