While everyone focuses on the talking heads (Zahn, Couric, et al) the hard fact is that these people are personalities, not necessarily decision-makers. Slamming them for network decisions is largely a waste of time, unless it adversely affects a large demographic. The real decision-makers are subordinates---obscure producers whose Rolodex is filled only with Republican names.
Their career may be on the upswing, and thus will do nothing to bite any hand that may someday pat them on the back. As a result, the perceived views of the Network are amplified and distorted into a bipolar view of the issues. Running the gauntlet of producers, assistants, etc often requires the skills of a seasoned agent--someone with time to invest schmoozing the proper people. And even they can occasionally get it wrong.
The Republicans have the advantage, however, in that they have multiple levels of contact: if Paula Zahn is unavailable, then her producer might be. If she isn't, then the booker might be; if she isn't, then they can talk to others up and down the chain.
Meanwhile, the Dems often sit and wait for the phone call from Zahn herself.
Any Big Media Network---whose politics leans toward the Financial Party--may become weighted with worker bees whose paycheck is tied to a perception. The process is self-perpetuating. As the producers lean conservative the organization becomes conservative. Liberals aren't booked because the booker has only conservatives in her Rolodex. If you swim in a red sea, everything you touch will be red.
Interestingly, the one thing with the greatest potential to change this picture follows the viral paradigm. As just about every Net-savvy person knows, a well-produced political video loosed on the Internet can reach nearly every producer in a media organization within hours. If the video is linked to a buzzword, the same subordinates within the organization who amplified and polarized the perceived and actual views of the company, now actively spread the counter-message. If the viral video is tailor-made for viewing within a specific organization, say, ABC or NBC, then it could result in significant changes in the perceptions among personnel in that organization.
Among the possible results: mid-level personnel within the organization may undergo significant change vis-a-vis important issues.
If the Network talking heads suddenly do an about-face and begin reporting the news from a perspective counter to their earlier broadcasts, I would bet that the proliferation of very subversive Internet videos had a lot to do with it.