That appears to be by design. I don’t think Trust Level lists are meant to be available to the general user base:
But the behavior of the visibility setting re. automatic groups is confusing, and has been brought up: Respect the visibility settings of all automatic groups - Feature
(So this might be a UX item rather than a Bug, but I’ll see if anyone else has thoughts.)
If you have a case where it makes sense to publicly list, for instance, a few TL4 users, you might find this theme component useful: Add groups to the /about page - Theme component
EDIT: I’ve just been informed that this component functionality is now in core, under Admin → Community → About your site:
