This is a way more complicated topic than it sounds. If there was a board dedicated to planning and carrying out mass shootings, or other violent acts, I'm sure

This is a way more complicated topic than it sounds. If there was a board dedicated to planning and carrying out mass shootings, or other violent acts, I'm sure it would get deleted rather quickly. But when the topic of a board is "discussion of politics", it isn't illegal in nature. It's the same reason why Reddit administrators hasn't deleted /r/the_donald yet, despite many of it's users breaking Reddit rules. It's hard to draw the line where it's acceptable to ban a whole community with "safe topic", because majority of posters are breaking the rules / law.