People don't bring up bands like metallica etc because you don't often see them singing about gangs and their lives in the 'hood'.
The members of metallica (and many other rock bands) are certainly not role models, however they don't go around promoting or glorifying the life of a drug abuser(as they are known former addicts and alcholics). They do have a few songs on the issue but they dont promote it. I dont see the comparison. That just goes into the debate of which music do you prefer, and this thread has nothing to do with that, these discussions ALWAYS go the wrong way. I personally listen to rock, and don't like rap, if I did, I still wouldn't be supporting a convicted rapist/thug.