Maybe it's due to 4chan's huge community size that fools are not a problem as they seem in smaller communities. Additionally, there is a whole slew of tools to identify (and punish) fools such as karma, comment voting, etc which perhaps weren't as widely available in the past.

4chan has mods who routinely remove child-porn and other questionable, non-related material. http://www.4chan.org/faq#whomod