You grant some rights to other users of GitHub, but I'm not seeing anything that prevents proprietary code:

> If you set your pages and repositories to be viewed publicly, you grant each User of GitHub a nonexclusive, worldwide license to access your Content through the GitHub Service, and to use, display and perform your Content, and to reproduce your Content solely on GitHub as permitted through GitHub's functionality. You may grant further rights if you adopt a license.

https://help.github.com/articles/github-terms-of-service/

To anyone more versed in US law than me, what usage specifically does "use, display and perform your Content" permit?

Yes