In my opinion banning should always be a last resort, with a couple of exceptions...blatant spam on the first post, threats of violence made to other members, posting porn (when that is explicitly not allowed), etc. And once the administration of a site is forced to use this option, a conscientiously run site will not have done so on a whim, but only after careful consideration and lengthy discussion among all the moderators/administrators. In some cases it may do more harm than good to publicly explain the details to the membership as a whole, as certain details simply need not be made public.
If I or a member of my staff is forced to ban an established user for repeated/habitual behavioral offenses, I will be irritated if another user openly questions me or a member of my staff about it (but I would not ban them, unless they repeatedly publicly posted about it after being asked to refrain from doing so). However, if I am asked privately about it, I will graciously answer the question, revealing only what it think isn't harmful to anyone involved, but I will try my best to be gracious and make sure anyone who has concerns knows it was not done on a whim, and was done because it was felt by the staff as a whole to be the best course of action. If enough people ask about it, I would consider posting an announcement to let everyone know that it was done only after due consideration, after many chances for the offending user to modify their behavior and as a last resort only.
I think if a site's staff maintains diplomacy and consistent moderating, and has well-defined guidelines for what is allowed and what is not, this kind of thing will be minimized. This conscientious moderating is the kind of thing you see going on here at CH...the moderating staff is a team, rather than a bunch of loose cannons going off at will like you see on so many other sites. Being a moderator is oftentimes largely a thankless job that can be unsavory at times. Moderating privileges should never be used as a means of getting the upper hand in a personal disagreement.