Networks in America and the rest of the world are owned by huge multi national companies like General Electric which owns cbs or nbc "I'm not sure as I'm aussie". They also own other companies that for example may produce something that may harm someone or something..
If the network reported the story it may leave parent company [General Electric] open to litigation or law suits .
SO G.E. may use internal pressure on nbc/cbs not to broadcast story . They also have the pressure of large advertising dollars to think about .
If you had a company threating to withdraw millions of dollars in advertising from your network if you broadcast a story that put them in a bad light , your network may pull the story to protect income . Not the truth
This is hypothetical of course but over the years there have been cases where other companies have used this tactic to stop a news story from being broadcast..
Public Networks like ABC in Australia are funded by the government with an [Hopefully] Independent board .
As they are publicly funded they don't owe allegiances to large companies
Also the freedom of no advertising means they can run shows that are not watched by the majority even though these shows eg: Documentaries , In depth Current Affairs , Arts ,Opera ,have great value but not a huge audience..
Hope this helps..