Why recommend GitHub for hosting source code, when there are (free/libre and) open-source alternatives like GitLab and Gitea? Those would be more in line with the spirit of the jam, I think.

Thanks, we updated the rules to additionally recommend gitlab, or any public git repo.

As for why we recommend GitHub at all, it's because that's the git host most people are familiar with, and the one most contributors will use, so they need to know it's allowed.  We take a progressive approach to introducing open source tools, a "when you're ready, check this out!" approach.