As many people already know, Craigslist is being sued under federal housing discrimination law for allowing the posting of allegedly discriminatory ads seeking certain types or classes of roommates and tenants (e.g., based on religious affiliation or sexual preference). Legally you can't post those kinds of ads in the newspaper and the newspaper that runs them is liable for housing discrimination. For those interested, here's a copy of the complaint (pdf format) against Craigslist.
Google, AOL, Yahoo, Amazon and other online firms recently filed what's called a "friend of the court" brief in support of Craigslist in the matter. As Craigslist CEO Jim Buckmaster points out in this Law.com news item, "Most of these companies/organizations have as much (or more) at stake in this matter than we do."
At stake is whether online sites should be compelled to actively police the postings of their users to ensure compliance with federal and state laws. The trademark policing issue in paid search is a cousin of this suit and raises some of the same general questions about the duties of online publishers.
That different treatment and outcome doesn't really make sense logically and will probably be changed eventually. But for now, if Goldman's analysis is correct, it looks the plaintiffs will fail and Craigslist will prevail.