> any idea about why these rules would be secret?

Don't work for youtube, but I'm guessing they would be secret for a couple of reasons.

1. If made public, people could post violating content and work around it by making sure they won't get flagged with carefully crafted thumbnails, content etc. Ex: Take a look at the 'Nudity' section. There are examples of 'Partial nudity' in which a guy wearing underwear is deemed inappropriate because it looks 'Vulgar'. So one could, in theory avoid such thumbnails, and still post vulgar content in the video and get by their censors.

2. It would also open Youtube to criticism, just like the Facebook breastfeeding content was, a whiles back.