In Section D.8 Private Repositories of the GitHub Terms of Service it says:

Paid accounts may have private repositories, which allow the User to control access to Content. GitHub employees only access private repositories when access is required for security or maintenance or for support reasons, and then only with the consent of the repository owner.

This paragraph defines when GitHub staff may access private repositories of paid accounts under certain conditions.
However, there are also non-paid accounts that can have private repositories, namely those given to students under the Student Developer Pack. It should be clarified that the protections also apply to those accounts.