“Just as we as a people are free to engage in objectionable speech — and to turn away from it if we are offended — hip-hop’s critics are free to debate, coax, bully, even legislate that speech. … But the bottom line in any commercial enterprise is what the market will bear. And here, a weariness with the mind-numbingly repetitive, casually hateful emphasis on violence, misogyny, homophobia, and commercialism may be doing the necessary corrective work.”