First, let’s note that most bannings (by a huge majority) are a single-post spammer, and no one ever asks questions about them. There’s no reason for us to post anything, and there’s almost never an email/PM question.
Second-most bannings are obvious trolls or returning socks. The obvious trolls, we don’t want to give them publicity. These are people who thrive on (“get their jollies from”) attention, even negative attention (clearly), so we don’t want to feed them by having threads about them or posts about them. Most posters are well aware of the troll-ness, so we rarely get more than a couple of PM/email inquiries. It may indeed require more work to answer email/PM inquiries, but we think that’s better than “encouraging” them by having threads about them.
The final situation are socks – people with multiple accounts. [ASIDE: if someone innocently created multiple accounts, we’re very forgiving once they understand our policy.] There are usually two types of socks:
(a) people who create multiple accounts to agree or argue with themselves. This is just trolling, and see above.
(b) people who were banned and are trying to sneak back in. If someone was banned for misbehavior and then creates a sock to try to get back in, we have no sympathy whatsoever. Sometimes we do announce these, but usually we (again) want them NOT to have publicity.
If someone is banned who has posted for a while, we usually do provide public notice. We generally don’t want discussion, because the person is banned and can’t defend themselves, so we think that discussion would be unfair.
While it’s not explicitly against the rules for someone to PM a moderator and then post the answer, it is counter to policy. We think the policy is a good one, for all the reasons listed above, and we would rather not have to deal with people violating the policy (on the grounds that it’s not technically in the rules.)
Does that explain more fully?