Clarification here: The ban list is public. The presently listed ban reasons are public. Anything beyond that, regarding bans, is comparatively restricted: the person who was banned may know and may share, but the staff will not.
As far as ban reasons appearing on profiles, this feature exists, but is presently staff only. I do not know the reasoning behind this, but it's probably due to ease of implementation. As such, while I'm not personally opposed to enabling it to all members, I also don't believe there's any particular rush to do so.