To prevent using them as a file host. If you can just dump files (of whatever size, it doesn't really matter) onto github and serve them natively across the web then they become a file share or a CDN. They'd have to seriously lock down the rules on publicly available free repos if that happened (and they'd have a copyright nightmare on their hand as well). Whereas if they force git into the loop on the client end then it keeps them in that repo hosting butter zone.