It can be useful (I've the case) to change users to display on a per admin basis (per data class).
I.e: display users of the group "bloggers" for the `BlogPost` object and users of the group "registered" for `PostComment`.

If we move this function to the ACL manipulator it can not be down without overriding the manipalator and testing the admin / data class type (not simple and a bit heavy for a so common use case).