That doesn't seem consistent w/ the intended usage patterns of git and github, e.g. github's 'Fork' button is displayed prominently on every src page, for the purpose of alteration and distribution of src.
That's the point. If you fork and don't do a pull request so I can pull it back, then you're leaving an altered copy out there. Can't do that. If however you do a pull request and I pull your changes in, then your fork is no longer an alteration.
This prevents people from butchering your work because they think they know better and then leaving half-assed junk out there to screw students over, but still lets people contribute fixes.
Although, there is one that prevents modification as well if that's what you're looking for.
For one, the book is targeted to Filipino students - teachers may want to modify the names/scenarios to better suit their country. Sure, gives them a low-hanging trolling fruit (e.g. putting vulgar words in the i18n section), but I really don't have the time to review these forks.