Board Thread:Support Requests - Community Management/@comment-452-20130610222845/@comment-1330470-20130610224509

"There are very few policies which apply across all Wikia. These can be found on the Central Wikia at Wikia:Category:Policy."

From c:starter:Project:Policy. It seems that Category:Policy is in fact intended to be global, or was at one point. Certainly this line inspired at least some wikis to act as you described.