I think this is a very interesting question, mainly because it's asking people to look beyond the obvious ... many (and I include myself!!!) are not fans of the man's views, but how does that translate into jokes about his death?

A short answer to your question: I think this is happening because most people view Rush as a symbol and not as a human being. For many, he simply stands as a caricature of all they dislike about the Republican party and/or radical conservative views. And he's certainly not blameless for this; in fact, much of his behavior suggests he thrives on being cast as the villain and inciting high profile controversy.

Thinking on this also made me recall a recent article on cracked.com called "What is the Monkeysphere" ... though satirical, I found it to be wise and you'll see a link to the full article in my sources below. The basic idea is that the human brain is only hard-wired with the ability to know and genuinely care about so many people as individuals--the article posed the idea that the number might be around 150. Beyond that, we may extend the goodwill we generally want to believe we feel about all of humanity, but we can't really empathize. For most of us, Rush Limbaugh (and the majority of celebrities) would be outside our "monkeysphere." Intellectually we know Rush is a real person, but we don't feel genuine empathy.

Jason Calacanis had a blog earlier this year on the darker side of the internet as a social world, and how easy it is to be cruel online to (or in speaking about) people who don't exist in our lives as real human beings. Some quoted bits from that article that I believe are relevant: "I’ve come to recognize a new disorder ... In this syndrome, the afflicted stops seeing the humanity in other people. They view individuals as objects, not individuals ... Today, we’re destroying each other with words, but teaching ourselves to objectify individuals and to identify with aggressors will result in more than psychological violence ... It’s only a matter of time, sadly, until this loss of empathy will hit the real world. We’re training ourselves to destroy other people, and there’s a generation growing up with this in their DNA. They don’t remember a world when communications were primarily in the real world." (See link below for full text of Jason's blog).

For all of us, the online world is still a pretty new thing and I think we're still trying to figure out how to behave ourselves and be good people in this environment. We are adrift in a world where we feel like we know people, but in truth we do not. Most of those we passionately view as villains and heroes exist only as symbols and/or mirrors of what we want to believe about ourselves.

To put it shortly, Democrats and those who aren't Republicans don't like him. Rush tries to be everywhere and many people don't like his thoughts and ideas getting into everything and everyone's thoughts.