You can't, because github hasn't made a mistake and accidentally made all private repos public.
If github screwed up one day and all private repos were temporarily made public it would be illegal for you to run a script that tried to scrape them all down to your personal hard drive.
False.
There are lots of things on webservers that aren't public. Try to access:
https://github.com/harryh/my-private-repo
You can't, because github hasn't made a mistake and accidentally made all private repos public.
If github screwed up one day and all private repos were temporarily made public it would be illegal for you to run a script that tried to scrape them all down to your personal hard drive.