Megan McArdle on Media Bias:

Bias matters not because liberals deliberately slant their stories, but because they are much more likely to interrogate the facts that contradict their ideological beliefs, than the ones that support them. When they come across an uncomfortable fact, they'll go out of their way to figure out why it isn't really true. When they come across a fact that confirms what they believe, they'll be more likely to accept it at face value.

I'm not claiming that liberals do this more than conservatives (I think that being human, they're equally prone to this phenomenon)--only that in the media, liberal bias is mostly what matters, because the media is overwhelmingly somewhere to the left of the American center. Even if you have a conservative reporter prone to insufficient interrogation of convenient facts, those same facts are going to set off alarm bells with his editors, who are quite likely to question the whole story.