For starters - not all African Americans can "get away with" making comments such as Shaquille O'Neal made. See the incident with Jesse Jackson calling New York "hymietown" in response to the number of jews who live in that city, and the resulting outcry.
I don't have an answer to your second question except to say that if you're a student of history, you'd know that it wasn't all that long ago that whites
were able to "get away with" the kind of humour against minorities you see on Comedy Central, Def Comedy Jam, and Comic View. A few references off the top of my head: the whole legacy of minstrelsy, black face, and Al Jolson being able to record "Mammy" and have it be a huge hit. The Warner Brothers cartoons that were done during WWII that featured absolutely awful stereotypes of the Japanese, or the cartoons that displayed Native Americans in a negative light..and don't
even get me started on Speedy Gonzales! Then there's the folk-opera "Porgy and Bess" (never mind that I actually like the show), and why is it that whenever Robin Williams wants to portray a particularly "sassy" black character in his monologues -- and it's usually a woman -- that he always lapses into "ebonics" or a black dialect, and no one so much as raises an eyebrow?
Perhaps the legacy of discrimination and disenfranchisement has something to do with it, but in my opinion, I honestly think that the majority lets comments like these that come from minorities roll off their backs because, at the end of the day, they're still the majority, still the power brokers, and still have the upper hand in western society.
Furthermore - the comedy that you see on cable television is largely performed before a same-race audience, and therefore, maybe the comedians feel that since they're among "their own" they have the freedom to say what they want. It's like Bernie Mac says - they say what folks are scared to say in real life.
But then, that's just what I think..I don't claim to speak for any black folks except myself.