From James Poniewozik:

As anyone following health reform knows, centrism is a political position too. And you see moderate bias - i.e., a preference for centrism - whenever a news outlet assumes that the truth must be “somewhere in the middle.” You see it whenever an organization decides that “balance” requires equal weight for an opposing position, however specious: “Some, however, believe global warming is a myth.” (Moderate bias would also require me to find a countervailing liberal position and pretend that it is equivalent to global-warming denial. Sorry.)

Often, moderate bias is just the result of caution, but the effect is to bolster centrist political positions - not least by implying that they are not political positions at all but occupy a happy medium between the nutjobs. Meanwhile, conservatives see moderate bias as liberal, and liberals see it as conservative - letting journalists conclude that it's not bias at all.