I just checked the admin CP, and it appears that the issue has to do with inheritance of forum permissions. If all members in the "registered" user group are allowed to post in the Political World, all of that forum's sub-forums inherit this, which means that a member of, say, "Socialist Only" will then have the ability to post in "Capitalist Only," and vice versa, because both of them are in the "registered" user group.
I'm not sure how to work around this, since the only solution I can think of right now is to set the permissions for each sub-forum individually instead of having them inherited from the Political World. This would be a major hassle and may contain unforeseen problems in turn, though, because it would entail manually setting up the permissions of every single user group for every single sub-forum of the Political World.
There are 35 user groups, 19 permission settings for each forum, and seven "Only" sections in the Political World. This means we would have to configure 35 x 19 x 7 = 4,655 manual settings in total. That would be an utter and unworkable nightmare, so I'm thinking about potential alternatives right now.