Why do they bother? Their news is the least successful of their programming.
I think they bother because they want to spread liberal bias and hurt the President and the Republicans, even if it costs them money and credibility.
The thing is, it has the opposite effect. It isn't subtle. It is over the top. It has been 15 years since this kind of thing worked, and only then becuase people had not yet become sensitized to the media bias. Those days are over.
The marginalization of network news will continue for 5-7 years and that will be it. News organizations that tell it like it is to a much more informed audience will thrive. Those riddled with "Bias" as Bernie Goldberg called it will be consigned to low circulation dailies (like the New York Times, Boston Globe and LA Times).
The result will be a better informed civic body.