And while I completely agree with your point that the project code can be seen as everyone's, how does some third party have the right to define how someone manages his or her own gate?

I'm looking at it more as, it's the maintainer's right to manage the project and it's issues as he or she sees fit. I can see how the maintainer doesn't have the right to manage the code outside of the repo, but telling someone how to manage their own repo...