Bias is a normal part of human thought. It is not a good thing, but it exists because we are only human. Our viewpoints influence how we see things. And that in turn influences how we describe them. But just because bias is normal, does not mean that it is acceptable.
A doctor may like one patient better than another. That does not mean that he has the right to provide an inferior level of medical care to one patient. He may not be able to help being nicer to one patient than the other, but he may not actively mistreat a less favored patient. That is medical malpractice. Similarly a reporter who does not simply favor liberal politicians, but actively biases stories against their opponents is guilty of journalistic malpractice. He can no longer claim to be providing a public service, only serving as the mouthpiece for his ideology of choice.