Why is it that comedians are allowed to make sarcastic, insulting comments, and no one else? So you aren't allowed to make funny, edgy jokes unless it is your proffession? Is that some sort of Comedians Union Local 238 protection racket thing going on?
Define allowed. As far as the law goes they're all allowed. Do you mean in terms of getting fired or getting public outrage?
PR executives of a large corporation are in a very different position from a stand-up comedian. They're a big public face both inside and outside their company and the things they say will be strongly coupled with the company's policies and disposition. A comedian isn't even necessarily working directly for someone at all, instead they're more likely to be freelance getting whatever jobs they want, and their audiences are pretty limited so they have a lot more opportunities to find work (if a star of a TV show tweeted this it's not out of the question that they'd lose that job)
The other thing is, if it's coming from a comedian people will be looking for the joke. Especially if it's coming from someone whose entire shtick is being satirical. Their entire job is to make people get the joke and make them laugh. The problem here is so few people seem to even realize that it could be sarcasm. I've been reading comments all over the place and I'm struggling to find anything beyond "stupid ignorant racist, fire her!", even on sites like Slashdot. People get that she's joking but that's about it, they all seem to uniformly think that the joke is at the expense of black people and AIDS sufferers. And it's hard to blame people since the tweet is very terse and confusing.
That said, comedians aren't totally immune from this sort of criticism. These internet scandals happen all the time and no one really cares about them for very long. For example: I remember people were outraged over this article a couple years ago (
http://gizmodo.com/5833787/my-brief-okcupid-affair-with-a-world-champion-magic-the-gathering-player) but I so vaguely remembered the details now that I had to google around a while to even figure out what the problem was. On the other hand, people will probably remember Michael Richards going on a racist rampage to offend some hecklers for as long as he lives.