Many of the folks signing and writing these songs feel like they are storytellers and they are reflecting society. They see themselves as something more akin to television or movie producers/writers than endorsers of the behavior.

Not dissimilar to Bob Dylan singing folks songs about characters of lower moral values. (Joey anyone? http://www.bobdylan.com/songs/joey-bob-dylan-and-jacques-levy )

So, if I'm a rapper and I grew up around violence I'm going to write about it.... just like a director might make films like Scarface], or the Godfather. Doesn't mean they endorse it, they just want to tell that story.

Of course young people listening to this stuff might have a different experience. Young people do model what they consume in media.

From NPR: "A recent study by the Prevention Research Center of the Pacific Institute for Research and Evaluation in Berkeley, Calif., suggests young people who listen to rap and hip-hop are more likely to abuse alcohol and commit violent acts. Ed Gordon discusses the issue with Denise Herd, an associate professor at the University of California Berkeley School of Public Health, and David Jernigan, executive director of the Center on Alcohol Marketing and Youth at Georgetown University."

http://resources.prev.org/documents/MusicSA_agression1.pdf

http://www.npr.org/templates/story/story.php?storyId=5390075

I don't think this stuff overwhelmingly pervades our culture, but it's so brash that it succeeds in getting attention like this. And, honestly, writing things about how immoral it is only makes it more notorious.

The majority of music does not condone degradation and murder, but the small amount that does gets all the attention, which is partly the goal, to be shocking.