On December 13, 2013, a verified user named Julie announced publicly the Friend Code Restriction, and that friend codes are not allowed on Miiverse. (Giving them out, asking for one, etc.) on this post. It was previously in the Code of Conduct, but it was only little thing in there and no one paid attention to it. This rule is hated by many. People don't like the fact that you can't share friend codes because you can freely send friend requests to people on the Wii U, but not on 3DS.
The restriction also applies to NNIDs, even though it's out in the open on someone's profile anyways. The admins also delete any codes that look like a friend code (to them) that aren't specified.