I can't speak for the moderation team's reasoning, but the inability to unsubscribe from hidden threads is a known issue that currently lacks a solution. Mahz said some time a while ago that he intends to put an unsubscribe option in the subs list, but there's no telling when that might actually happen due to his decision to not push through upgrades while he's too busy to reliably get around to fixing horrible bugs and problems that might crop up with new features. You'll just have to deal with it for now, unfortunately.