The Nielsen ratings timelines show a fact not a motive.
It is factual that viewers of one channel tuned out during an opposing viewpoint, viewers of the other two channels did not tune out during an opposing viewpoint.
Your assertion may be why MSNBC/CNN didn't tune out during house presentations, but doesn't explain why they didn't tune out during defense presentations.
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Meanwhile, research on "approach-avoidance" behavioral patterns suggests there is a partisan difference in the degree of partisanship one needs one's sources to exhibit:
There's also good evidence that this kind of "selective exposure" is disproportionately powerful among Republicans. "One study found that adding the FNC logo to a news story increased the probability that Republicans would choose to read the story by 25 percentage points, whereas adding CNN’s logo or NPR’s logo reduced the chance by 10 points," Grossmann and Hopkins write. "No equally strong effects occurred among Democrats."
http://pcl.stanford.edu/research/2009/iyengar-redmedia-bluem...
The concern I'm raising is not that, but this:
"Thus, as the audience become polarized over matters of politics and public policy, rational media owners stand to gain market share by injecting more rather than less political bias into the news (Gentzkow & Shapiro, 2006). ... A further implication of voters’ increased exposure to one-sided news coverage is an ‘‘echo chamber’’ effect—the news serves to reinforce existing beliefs and attitudes."
The system needs a way to systematically correct for this.