The fact that the Github license allow forking does not mean all sources their are free to copy and use.

Github's response to this question (2013 already):

Pavlo Mykhalov‏ @growtofill 13 Jul 2013

What license terms are applicable to a public @github project if none are specified within the project?

GitHub Support‏
@GitHubHelp

Replying to @growtofill
@growtofill We don’t impose any licenses on repositories. You should ask the owner to apply an open source license if you want to use it.
7:32 PM - 13 Jul 2013