"Prosecutorial discretion" is when a prosecutor makes prudential decisions about what crimes to prosecute. Perhaps he goes after more drunk drivers and fewer casual marijuana users. Perhaps he targets spousal abuse and ignores minor property crimes such as vandalism or theft from garages.
The journalistic equivalent is "editorial judgment," in which journalists and their editors decide what stories to cover. The problem with editorial judgment, like prosecutorial discretion, is that it can too easily become bias. If a prosecutor targets certain crimes, he might sweep into law enforcement's purview more blacks and other minorities; if a journalist targets certain stories, he can too easily become a Democratic Party shill.
In the Internet age, editorial judgment is even more problematic, because the capacity exists to provide the public with the primary documents (or videotapes) and let the public itself make its own judgments. Consider, for instance, the circumstance of Mother Jones magazine. It obviously has video of a Mitt Romney fund raiser. It has chosen selectively to release two snippets from that fund raiser, regarding Romney's comments on people who don't pay taxes, and Romney's comments on the Israel-Palestinian conflict. Why doesn't Mother Jones simply place the whole video on its website so we can see the full context? Doesn't their failure to do so show that they are not acting as journalists, but simply as Democratic Party hacks?