As said, the problem it technical more than anything else. We try to keep the forum structure simple, to avoid that if we want to make a minor chage we must spend a day to check all groups.
This means that gorups have rights in boards that they can see. As unban requests are by nature obliged to be seen by all, to separate rights would mean a very complicated structure.
Besides that, as moderators can not ban them selves, they should not be able to post.