I mean, "everyone already has an account" is already a very good reason. That doesn't mean "I automatically accept contributions from everyone", it might be "I want to make the process of contribution as easy as possible for the people I want as contributors".
Hatching a reputation-based scheme around a "Contributor Management System" and getting "the people you want as contributors" to go along with it is easier than getting them to fill in a 1/username 2/password 3/confirm-password form? Choosing to believe that is pure motivated reasoning.
People aren't on Github just to implement reputation-based management, though.
What does that observation have to do with the topic under the microscope?
> GitHub customers really are willing to do anything besides coming to terms with the reality confronting them: that it might be GitHub (and the GitHub community/userbase) that's the problem.
The community might be a problem, but that doesn't mean it's a big enough problem to move off completely. Whitelisting a few people might be a good enough solution.