Some shows go too far and cross the line. Carlos Mencia, and Sara Silverman seem to just be all about pissing people off and shock value. The writers for Drawn Together seem to just arbitrarily pick obviously sensitive subjects out of a hat and make insensitve references and shock-gags based on them. When you watch you aren't laughing because it's funny, you're laughing because it surprises you and makes you feel uncomfortable. It seems that even Family Guy is going too far now to try and stay ahead of the curve - some examples being the "prom date dumpster baby", and "you've got AIDS" songs.
The difference is that a lot of these shows don't have any real comedy, just shocking statements and gross-out humor. Often times the shallow storylines simply allow for this humor and are secondary to the concept of the show.
Even South Park with it's absolutely abhorrent and tasteless humor has a good meaningful moral at the end of most episodes!