Well, if you create a message board that accepts images, and you make it so there is no technical way to remove those images, you are implicitly including child porn in your definition of free speech.
It's typically included on lists because of the implications. One either takes the position (1) child pornography should be allowed to be shared, (2) child pornography should be suppressed, but may be shared, or (3) child pornography cannot be allowed to be shared.
(3) obviously requires very different technical solutions (like: no private encryption, ever) than (1).