This part of the ToS explicitly does grant such a license to other GitHub users.
> If you set your pages and repositories to be viewed publicly, you grant each User of GitHub a nonexclusive, worldwide license to use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub's functionality (for example, through forking).
I was thinking that. The language seems not quite clear, at least to a non-lawyer like me:
> you grant each User of GitHub a nonexclusive, worldwide license to use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub's functionality (for example, through forking)
So, on GitHub, we can "use", "perform", and "reproduce". Does editing/modifying fall under any of those verbs?
Cloning is part of GitHub's functionality, so therefore we can clone it but can only commit those changes back to GH. We are permitted to do anything that is part of GH's functionality.
> If you set your pages and repositories to be viewed publicly, you grant each User of GitHub a nonexclusive, worldwide license to use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub's functionality (for example, through forking).