I haven't heard Diane Rehm's program.
It [NPR] serves a population which is going to be more liberal than the average bear-- namely, people who give money to public radio/TV and who style themselves smarter. Or in other words, the liberal arts side of the upper middle/lower upper class.
I have given money to two public-broadcasting stations but would not be considered liberal, nor upper-middle/lower-upper class. As for styling oneself smarter, well, we're all guilty of that now and then.
It seems to me that they've made an effort to be more balanced of late, though they do slip. I remember a story (Morning Edition) on the missionary activities of the African Anglican bishops in the USA which had Bishop Bennison talking. What they didn't say was that Bennison is most definitely a party to the dispute and should thus not have been introduced as if he were a neutral commentator. But I think that was a slip rather than actual bias per se.
Probably bias. Chances are both NPR and its listener base, the folks who fancy themselves a liberal elite, agree with Mr Bennison on the controverted issues, such as gay marriage, lady priests and radical revisionist theology/biblical studies.
(Editorial: That isn't religion but self-centered 'spirituality' - the kind of thing this elite likes. The conservative, smart and very funny P.J. O'Rourke once wrote that when hardship hits, people drop that fast and turn to RELIGION. How many people have had deathbed conversions to/reconciliations with some New Age guru?)
And they hardly show the kind of axe-grinding cant of, say, Rush, whose purpose seems largely to be to give engineers and computer geeks a harmless outlet for their (mistaken) notions that they can run the world better than anyone else.
I haven't heard anything from Rush Limbaugh in about 6 years but from what I can tell he is basically a pro-war shill for the Republican Party.