So as long as the FCC allows such talk on the radio, and Rush has an audience that wants it, and advertisers who'll support it, he will continue. That is the price of freedom of speech. Sometimes people say things that are offensive. A person with any real class wouldn't have to do that in order to have a successful radio show, but there will always be those who will stoop to a low to gain ratings and popularity.
Actually, you make three important points:
1. About the-FCC-permitting-such-talk: The FCC really DOESN'T want to get involved with content, as you'll hear a commissioner explain: http://getonthenet.com/McDowell.mp3
2. About Limbaugh's audience: A 5 Share makes him the biggest star in Talk Radio. The other 95% of listening is elsewhere.
3. Bring up The First Amendment, and, invariably, someone will talk say you-can't-get-a-little-pregnant, and someone else will bring up shouting-FIRE-in-a-crowded-theater. It's only CALLED "Free Speech." The cost is that everyone-gets-to-talk. Limbaugh, Skinheads, the Klan, Anita Bryant, everyone. The real freedom is freedom-not-to-listen.