There are drawbacks to this discretion. Firstly, it gives people the impression that "nothing happens", that no moderation is taking place. You see some abuse somewhere, you assume that it's deemed okay and that the author is simply taking a breathe before the next abuse. And so, you react to it as to an ungoing, unresolved matter (in a way, "waiting for the second shoe"). Secondly, there is no exemplarity. With general rules, boundaries are tested and defined by practice. And this invisibility means that the testing is being re-made individually every time : nobody knows where exactly are the walls they'd bump on.
This is why, on a different scale, legal judgements are public. They give closure, and they exemplify boundaries. And this is why on many forums, not only bans are explicit (preventing people to pointlessly address absentees) but are given a reason, be it technical ("user banned for post #3586", "user banned by rule C§41 ter") or more sarcastic (the personalized penny arcade notifications are hilarious, echoing the offense and/or the user signature, while still giving a precise idea of what is not acceptable).
It's a domain where transparency (clarification of what is going on where why how) can snowball for the better. Partly because the forum is bigger than the four or five users that can be "schooled" separately, and, thus, would benefit from indirect moderation of practical exemplarity.
You would avoid forumgoers (old and new) re-discovering one by one what these rules intend to mean, and you would prevent wrong assumptions on "what" is moderated or not, "who" is moderated or not, etc.
I am in agreement with all of this.