In support of the moderators policy though, where would they conceivably post an explanation of the ban?
People usually don't get banned permanently for a single post, but for a number of infractions of PPRuNe policy, and those infractions may be in multiple threads and even in multiple subforums.
Other than creating a special moderator-post-only "tar brush" subforum somewhere (next to Jet Blast perhaps), I can't think of a logical place.
Apart from the fact that disclosing why someone got banned will stir up a discussion every time. Possibly leading to a rehash of the things said that lead to the ban in the first place.
So even though I'm not 100% happy with the current solution (and realize that that's my curiosity speaking) I can't see the moderators doing otherwise.