Dang, I was going to try to refute you by saying no license meant no restrictions, but nope! Creators of software (and any written work apparently [1]) are automatically granted copyright for their work (this makes sense) so without a license, copying it would be a violation of copyright. That's wild, but seems reasonable.
Posting it to Github isn't implicit permission, but it would probably be a factor if the author did try to sue people for using his or her copyrighted work without license/permission.
There's also apparently terms in the Github ToS that allow viewing and forking of public projects regardless of license. What this means in a practical sense, I'm not sure.
It's pretty simple. In a practical sense it means that the author of any license-free project on Github who finds their code used in other software can, very cheaply, C&D the authors of those packages.
Posting it to Github isn't implicit permission, but it would probably be a factor if the author did try to sue people for using his or her copyrighted work without license/permission.
[1] http://en.wikipedia.org/wiki/Open-source_software#Open_softw...