There was a recent injuction against Limewire that may shed some light on what is happening with the adult classified websites. The adult classified websites have operated under the theory that they were not liable for third party submissions or content from advertisers. I'm not a lawyer, and maybe a lawyer here can give more insight, but it appears that the court ruling against Limewire may have set a precedent challenging that theory .
Here is a link to the injunction documents.
http://download.limewire.com/injunction/Injunction.pdf
Limewire had used a similar approach as to their liabilty, "we are just the middleman and can accept no resposibilty for what our users do legal or illegal". The timing of the injunction against Limewire and the timing of the adult classified sites action seem to coincide. And hey, let's through in an election season to add some extra heat and publicity.