User blog comment:Semanticdrifter/Updates to COPPA/@comment-5716479-20130624204026/@comment-452-20130624213341

If a website's "primary target audience is children", then the website operator may not collect identifying information from them.

IP addresses are regarded as identifying information.

Therefore, Wikia may not collect and share IP addresses from users of websites whose "primary target audience is children".

Since IP addresses are recorded as shared if you edit while logged out, wikis whose "primary target audience is children" trigger COPPA.

So, Basically they disabled anon editing for all major wikias whose primary target audience is children. - as is necessary to comply with the law.

If you're a lawyer who thinks that Wikia has somehow misinterpreted that law, I suggest you contact Wikia and offer them your services.

edit: see also, Brandon Rhea's comment below.
 * We would also be violating the law if we collected certain private information on wikis directed to children