By setting your repositories to be viewed publicly, you agree to allow others to view and “fork” your repositories (this means that others may make their own copies of Content from your repositories in repositories they control).
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). […] If you are uploading Content you did not create or own, you are responsible for ensuring that the Content you upload is licensed under terms that grant these permissions to other GitHub Users.
I wasn’t aware the GitHub terms of service explicitly grant / require you to grant permission to fork [within GitHub].
GitHub ToS section License Grant to Other Users