alt Hacker NewsI 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.