Nisbet finds the bias against news bias to be “troubling” because partisans fail to perceive bias in news that is in fact biased.
Explanation:
Matthew C. Nisbet says that partisans in an attempt to criticize news biases has failed to perceive real biases in the news. The partisans are so much inclined towards finding whether media houses are giving coverage to their ideological interests that they fail to perceive real biases by the media houses.
In the current political climate, Nisbet feels that the competition to criticize one another's ideologies by partisans have caused polarization of trust in news houses.