This is the cancer at the heart of news reporting. Not "libertard/conservatard media bias" (barring FOX), not "the manufacturing of consent" and not even evil reptilian shapeshifters from the 4th dimension trying to keep Teh People down.
Nick Davies, who is currently ripping Rupert Murdoch and News International a new asshole over the phone-hacking scandal, lays this all out quite clearly in his book Flat Earth News, published way back in 2007.
The problem comes down to this: news is a business. In the past, making a profit was an acceptable, nay, good business practice. Nowadays, you have to bring back increasing profits every financial quarter.
What this means is, is that you have to do more work than ever, relying on less people than ever, shifting more stories than ever. You're instantly reacting, instantly passing along the key information, writing it up and getting it on the screen and on the website. And you gotta do it, in many cases, in 10 minutes or less. No time for introspection. No time for fact-checking - just trust the newswire and whoever their source is, even though newswires openly state they do not undertake fact-checking of any kind. Don't follow-up, or do an in depth report - you've got another 15000 stories pilling up in your inbox!
So you gotta grab market share. That means you have to make the news stories dumb. You've gotta pander to this view of the "stereotypical, average viewer", which is often an offensive atrocity in and of itself. You gotta slot the story into neat little worldviews.
You end up telling people what they want to hear, rather than what people actually need to know, because, as Kai says, people love white sugar, not nutritionally balanced diets.
And all this allows the news to be manipulated by people who are politically motivated, who are passing disinformation down the line, who are looking to make a buck off your ignorance. Newswire gets a story from an "anonymous White House source". They write it up. They pass it along the line. The newspapers pick it up uncritically and published it. Did anyone check the source's information, whether or not this source has a history of passing along good, solid information, whether they're a political appointee with an axe to grind? Fuck no. And that's how you end up slipping in bullshit about WMDs and fictitious terrorists into the mainstream media and thus nationa consciousness.
Throw into the mix a politically motivated station who uses controversy and inflammatory tactics to drive up their viewer numbers, and you have the Perfect Storm of Media Dumbassery. They're chasing numbers, that's all. Newspaper sales and viewers. They'll do whatever it takes to get them because that's what brings in that quarterly growth. And if they have to ape the tactics of a station that is patently a political disinfo outlet to do it, they will.
CREAM, baby, is what explains the media. Understand the business model, understand the game.
Cain ... cutting through the bullshit, as ever!
ETA: change the words "news" and "story" to "media" and you expand the scope