There's an ideological edge to this laziness: Reporters get their pre-fabricated stories from lobbying groups, politicians, and pressure groups they are predisposed to trusting, and all of these groups lean to the left (or are outright self-identifying as progressive).
The easiest story for a reporter to write is the one that is basically dropped on his desk. To work to get "the other side" of the story, and truly vet claims, would take, well, work.
Hypothetically: Groups aligned with Justice4Trayvon can tip a reporter that there is video showing George Zimmermann looking reasonably healthy as he enters the police station after the attack.
These groups probably already have the tape themselves.
It's the easiest thing in the world for ABCNews to now run that tape. Oh, maybe they have to hide Zimmerman's headwound a touch.
But otherwise it's easy.
What would take work is to determine Zimmerman's appearance immediately after the beating -- that's not hand-delivered to them by a Justice4Trayvon pressure group, so it would take some work to get the pictures of a battered and bloody Zimmerman from the prosecutors' office. You'd have to do a lot of source-chiseling and favor-trading to get those.
And work is hard. So they don't bother. They have the easy story. Why bother with the full, accurate story?
In addition, it's always the case that agreeing with the herd is the safest position to take. Political correctness is always a good safe harbor for the stupid or uninformed, and every reporter knew that in this case that they would not be challenged nor would they be subject to the scorn of fellow members of their class if they simply went along with the script and demonized George Zimmerman. If they were wrong, they would all be wrong together, and hence they could not suffer, in relative terms, compared to any other member of their class.
On the other hand, pushing back against the demonization of George Zimmerman entailed risks.
Yes, as the media often claims when defending itself, it is indeed incompetent, ignorant, and lazy. Yet if there were not any underlying political bias we'd expect them to be wrong to the benefit of either "side" more or less equally.
But that's not the case at all; their errors run to the benefit of a single "side," consistently.
And that's because ideology tells you what to think when you don't quite no what to think; the more unsure of the facts someone is, the more he relies on his basic worldview and ideology as a first approximation for the truth.
And in the case of the media, being ignorant of a great many things, that first approximation usually becomes a second and third approximations as well. And all of those approximations are guided by liberal ideology.