at a minimum: when Rock did it, people laughed because there was some wit behind it. have you ever seen those routines of his? i think they're quite funny. as a white person i don't feel oppressed or mortally insulted. i don't feel that Rock hates me or whites in general. by the way, Rock also goes after blacks - they're certainly not spared from ridicule.
Michaels (Kramer) did nothing but yell "nigger" to black hecklers, out of sheer frustration. he didn't have a good come-back (because he's a lousy stand-up) so, caught without defense, he bared his true feelings. no one laughed because there was nothing to laugh at. it wasn't casting racial stereotype in a humorous light, it was naked racism.
i'm not addressing the wider "is it ok to be black and racist" question, just sayin' this is a bad example to use
(aside: i do love it, though, when people take a wide social phenomenon and try explaining or countering it with anectodes)