I think it might have something to do with the email verification. I had perused these forums for a couple months without ever seeing any spam (before I joined as a member) When I finally decided to join the forum, I wasn't able to because the email verification never arrived. After lots of digging around I finally found an admins email and let him know I couldn't join. He worked his "admin powers" and verified my account allowing me to post.
Shortly after I joined, like within a day, I noticed new members popping up left and right, as well as the beginning of the spam. My guess is that they ditched the email verification since it wasn't working (for me anyway), but that's still just a guess.