The GitHub repositories integration (“See all your GitHub repos”) requires write permissions for public SSH keys, as well as access to private repositories. Having both non-optional renders the GitHub integration unusable for quite a range of people: The first for i.e. anybody who has some responsibility with regard to 3rd parties (i.e. contribution privileges in organizations), the latter for anybody who just does not want the application to access private repositories.
As I understand it, the whole github/bitbucket authentication process is optional. you don’t have to give it permission to do anything but access your repos. Just create a cloud9 account, manually add the ssh key to github/bitbucket, and away you go. It’s a convenience factor imo.
Fair enough, I do not claim the the integration feature being required to work with GitHub. But the convenience of having the non-sensitive features (incl. any future ones) in there would be quite nice,.