No it isn't. Olbermann's contract was not renewed because Comcast is not Government Electric (GE), so they don't want the embarrassing and shrill far-left commentator Olbermann on their network. He will likely be replaced with a more moderate liberal.
I hate to break it to you, but in the days of the Unfairness Doctrine, Keith Olbermann would have had as much chance of getting on the air by himself for an hour as Sean Hannity or Bill O'Reilly.
Conservatives such as Lou Dobbs, Glenn Beck, and Tucker Carlson have been forced out at CNN and MSNBC in the last few years. 2 out of 3 have moved over to Fox.
Fox is conservative, but CNN is clearly a center-left network and MSNBC a hard-left network. Among the business news channels, CNBC is liberal, while Fox Business is conservative.
When you say that there is "no evidence" that journalists inject their political bias into their reporting, are you familiar with the Fox News memo that liberals were complaining about a month or 2 ago? Even Fox News has to inform its reporters that they need to avoid taking a side on the global warming debate and to report both the alarmists and the skeptics with equal weight.
In the case of talk radio, the reason why liberal talk radio is virtually non-existent is because liberals won't listen to talk radio. There was a liberal network called Air America that failed a few years ago, due to lack of listeners. If you want talk radio hosts who reflect your political views, you have to listen to those hosts to keep them on the air. Even fringe conspiracy theorists like Alex Jones (his shows are centered around 9/11 conspiracy theories, New World Order conspiracies, Birtherism, and other similar theories) perform better than liberal hosts.
With the exception of Fox, just about all of the news networks on TV are either "centrist" or liberal. Most of the television programs have subtle or no-so-subtle left-wing biases. The conservative base is much larger than the liberal base, yet the apathetic middle (which tends to decide elections), votes heavily for Democrats. Presumably, most of these apathetic individuals allow the media to tell them how to vote. In general, the least informed elements among the electorate tend to vote liberal and the well-informed voters are disproportionately conservative (one proof of this is the "gender gap," as men are more likely to be interested in politics than women and tend to be more conservative).
Your view regarding the "evil corporations" being conservative is not accurate either. For the most part, corporations tend to favor big government and are usually liberal. Most of the wealthiest Americans are liberals (Warren Buffet, Bill Gates, George Soros). The articles attacking the Koch brothers (who are 2 of a small number of wealthy people that are actually conservative) were written by liberal news outlets seeking to try to turn other wealthy people against the Kochs.