On the other hand, it's bad to assume that because "everyone uses it", it's OK.
Having dealt with people who send code as HTML email with quotes replaced with "smart quotes", having been sent links to files / tarballs that don't actually link to those file / tarballs but to a web page instead, having been sent to sites that require knowing how the site works and/or browsing around and being unable to use a link to download directly via curl / nbftp / wget, I can understand that a group that doesn't want to deal with superfluous distractions would have rules about what can and can't be used.
GitHub is NOT trivial, and it is extremely browser centric. Trying to just download a file is an exercise in frustration if you're not familiar with GitHub. Copying a link to your clipboard that can actually be used to download a file is next to impossible, unless you already know how to use GitHub.
I can fully understand why the competition organizers specifically forbid the use of GitHub in their rules.