You do not sue somebody for libel unless they had the power to not commit that libel. By giving the responsibility to the ISPs for the content they cover, we have also implicitly granted them editorial control over all the content that flows through them. Read that again; there's nothing that limits this power to "only things that might be illegal", as that's just not how it works... there's hardly a <META HTTP-EQUIV="illegal" CONTENT="true"> tag on all illegal speech! We grant great power to the ISPs as well as great responsibility.