Most people don't seem to have a problem with the radio playing, music or news or talk radio doesn't seem to require attention that is normally focused on the road.
My completely uneducated guess on this one is that listening to the radio requires very little mental bandwidth. If you're listening to music that you hear frequently, your brain isn't doing any processing because you already know the song. For news or talk radio, your brain probably isn't spending the extra processing time needed to commit to memory anything that's said.
In contrast, conversations with other people (whether on the phone or in the car with you) require enough processing for you to be able to respond intelligently, which distracts you from focusing on driving. At least if the other person is in the car with you, they're also able to spot sudden dangers or recognize dangerous situations where it would be better for them to stop talking.