It's the default on new accounts for stuff when people do things through the GitHub interface.

If you set user.email using git-config on your machine to a real email address and decide to author and publish commits with it, then GitHub will, of course, not be able to stop you (aside from maybe rejecting the commits when you tried to push them). It can't just arbitrarily rewrite the email address in the commit. That would break Git's data model.