it's not that it's bias, it's cowardice. From the Center for American Progress:
Think Again: NPR: Still Bending Over Backward:
"What is really going on here? Why would NPR, home to so many liberals and, one presumes, quite a few gay people, bend over so far backward to offer credibility to something associated with conservative efforts to discriminate against and persecute gay people? Alas, it’s a familiar syndrome. Journalists have been so cowed by conservative attempts to “work the refs” that they end up writing reports that raise the question of whether President Obama might be a Muslim or whether global warming is a conspiracy of a power-hungry mad scientist.
A particularly egregious example of this tendency occurred last year on NPR when it saw fit to quote the discredited right-wing zealot David Horowitz in an obituary for leftist historian Howard Zinn. 'There is absolutely nothing in Howard Zinn's intellectual output that is worthy of any kind of respect,' he explained."