It's a poser

Can anyone think of an interesting policy justification for why my posting defamatory material in someone else's blog comments does not create a possibility of them being held liable even if they ignore notice from the person being defamed; but my posting a copyrighted mp3 or a link (I'm pretty sure) to a copyrighted mp3 would lead to the blog owner being held liable if they ignore a proper take down notice? That is, why do we value intellectual property more highly than avoiding torts committed against people, even fairly serious ones? For an example of a serious one, see Zeran v. AOL.

This question was posed in my last class, but not answered. The class also featured a student telling Jack Balkin about The Superficial, and Prof. Balkin sounding quite interested.