This shocker is because of a little-known practice in broadcast syndication called a "barter deal." (Barter deals were briefly mentioned in Michael Wolff's first-rate recent piece on Rush in Vanity Fair).
Here's how a barter deal works: To launch the show, Limbaugh's syndicator, Premiere Radio Networks -- the same folks who syndicate wingnut du jour Glen Beck -- gave Limbaugh's three hours away -- that's right, no cash -- to local radio stations, mostly in medium and smaller markets, back in the early 1990's.
So, a local talk station got Rush's show for zilch. In exchange, Premiere took for itself much of the local station's available advertising time (roughly 15 minutes an hour) and packed the show with national ads it had already pre-sold.
Think Gold Bond Medicated Powder.