Why do journalists expect to have credibility?

Clark Hoyt, New York Times Public Editor, March 21, 2009:

THE Times has a tough policy on anonymous sources, but continues to fall down in living up to it. . . .Last year, at my request, a group of journalism students at Columbia University studied anonymous sources in The Times and concluded that their use was actually down by roughly half since a strengthened policy was adopted in 2004. But the students said the paper failed to follow its own rules for explaining them nearly 80...