Section 5 (License Grant to Other Users) of GitHub's ToS currently grants "each User of GitHub a nonexclusive, worldwide license to access [anyone else's] Content through the GitHub Service, and to use, display and perform [anyone else's] Content, and to reproduce [anyone else's] Content solely on GitHub as permitted through GitHub's functionality".

The problem I see with this is that it doesn't grant other users of GitHub a license to open pull requests as pull requests require modifications to be made in the fork. The rights mentioned in GitHub's ToS only allow use of GitHub's functionality when one only accesses content, uses, displays, performs, or reproduces it. It doesn't grant a right to modify other users' content.

As pull requests are an important part of GitHub's functionality, I think it's important that GitHub's users can be sure they won't be sued when opening a pull request to help improve a project without a license more permissive than the default license required by GitHub's ToS.

It seems unlikely that this would happen, but given a long enough timeline, anything with a non-zero probability will happen, and a single troll possibly suing themself could seriously harm both GitHub and the projects hosted on it, as some users would be too scared to open pull requests on repositories without a license more permissive than the default license or with a license they don't understand.