Brian Doll, GitHub’s VP of Marketing, confirmed this arrangement is intentional:
Code without an explicit license is protected by copyright and is by default All Rights Reserved. The person or people who wrote the code are protected as such. Any time you’re using software you didn’t write, licensing should be considered and abided.
Ironically, this situation exists because the founders of GitHub want to ease code sharing. They were worried that selecting a license for a new project was so difficult that requiring new project initiators would be a barrier to the adoption of GitHub
(Full Story: GitHub needs to take open source seriously)