Because in this case and most others (but not all cases) it's the publication/sharing of the content that is the problem. An advert for a slave isn't an issue

Because in this case and most others (but not all cases) it's the publication/sharing of the content that is the problem. An advert for a slave isn't an issue if you write it, print it out and keep it in a draw. Likewise you can produce as much blasphemous, libellous, and obscene content as you like but if you don't share it with anyone, no one will care (unless you broke some law in creating it - stolen information, child pornography etc.)