The Left can only make non-substantive attacks. Sure Limbaugh's comments included a personal slur but his attack in the main was substantive and the loony left doesn't and can't refute it. The Left doesn't have a substantive leg to stand on so they'll attack collaterally instead. All they can do is take a comment out of context, beat an emotional drum, and chant - this time it's "Stop Rush."
Captures them in a nutshell - the so-called Intellectual Progressive movement is really irrational and degenerative.
I am not in agreement with these advertisers, but Rush has gotten more rambling and irritated over the years. He is not the same funny and light hearted Rush and I think the left has gotten to him a bit.