Why is rap(black music) criticized for its violent/ual/drug content but heavy metal(white music).....?

That's not entirely true, remember when Marylin Manson got sued because some kid went out and did something stupid and said it was because of Manson's music? It's happened to other artists too, but that's the one I remember right now. I think rap's issues are a bit more public because metal is a bit more close knit.