It's relatively normal for a site to claim a permissive license for the purpose of implementing site functionality, such as transmitting, displaying, rendering, or backing up the content. It's far rarer for a ToS to claim a permissive license for arbitrary other purposes, let alone grant that license to every other user of the site. The ToS changes here grant such a license to all GitHub users, and not just to use site functionality (e.g. hitting the "fork" button).
The few other times I've seen a site attempt to claim such rights over user-submitted content, it has resulted in a backlash just like this.