I sold advertising on a heavy metal station back in the late 1980s. We were rated #2 in the 18-34 timeslot, a highly desirable demographic. But, our ad rates were less than our competitors who were at #3-5. During that era, advertisers believed that our listeners were slackers without money. Even though we had good ratings, we could not charge as much as some radio stations because of our format. The demand for our ad slots wasn't as high as it was with more mainstream, innocuous pop stations.
Also, during this time period, Slash was interviewed by one of our DJs during the morning drive. This was a major coup, GNR was at the top of the charts back then. But...Slash said the F word several times, and the idiotic DJ didn't have him on time delay. Not only did we get fined by the FCC, but a couple of my advertisers pulled their ads (money out of my pocket and out of the station's coffers). The DJ was later fired.
It's not just about ratings. The image of the station is everything. In beck's case, he's increasingly presented an image that isn't desirable to most advertisers. He appears fringe, offensive, erratic, and frankly---crazy. He's the Charlie Sheen of political talk right now. That's not a good thing. Major corporations want to advertise with programs that aren't going to offend their potential customers.