You are not allowed to Slander on the airwaves according to FCC Rules. Now one can make a case that telling the Truth about someone is not Slander BUT Courts and Juries decide such. The FCC can come down hard on stations for even the slightest rule infractions and heap on huge fines and even pull Broadcast license.
Money is the issue and the FCC can hammer the wallets of all involved legally. (Notice I said "legally" and not "morally right")
Rush did not slander anyone. And if you ever listen to any radio program the talking heads are always bashing on someone.