> third parties, who are not involved in the illegal activity have a right to discuss that activity
What you're missing here is that they don't have a right to make money off of it. RedBook weren't just facilitating the discussion of illegal activity, they were making money out of the advertisements on the site. They were directly involved in the illegal activity.
Actually, advertisements were free. They made money on enhanced reviews of providers and clients. In other words, one could, for a fee, dig deeper into reading comments between and about users of myredbook.
The owners of myredbook were still a third party in these comments and did remove postings that could encourage child prostitution, robberies, or other serious violent crimes.
What you're missing here is that they don't have a right to make money off of it. RedBook weren't just facilitating the discussion of illegal activity, they were making money out of the advertisements on the site. They were directly involved in the illegal activity.