tl;dr: Private is easier/cheaper.

Almost any set of policies and procedures can be gamed by a motivated set of participants, and the more information that is exposed by a moderation process, the easier it becomes to game the process, and the harder/more expensive it becomes to support a moderated environment, whether by volunteers or paid staff.

Public activity logs, detailed policies, and detailed procedures would be an amazing expression of transparency that would then immediately become a complicated house of cards requiring constant maintenance, defense, and review. For volunteers that is either a huge time-suck or a high threshold. For paid staff, the same, plus expensive, plus a target for law-suits.