Let's do a thought experiment. Start with a blank piece of paper and some colored pencils. A person begins drawing a picture. The page begins as a completely meaningless object, and as marks are made on the page, it gains meaning gradually. A line on paper is not illegal, or at least it shouldn't be by any moral or ethical standard. Two lines, three lines, and so on. Each are probably completely innocent individually. If these scribbles were forming letters and words, they would be clearly protected expression, until they formed some kind of credible threat. At least, that's how I understand it.
But this isn't a written message, just a picture. A head takes shape. Eyes, nose, mouth, and hair. The subject starts to emerge. Still this is a legal drawing by any measure. Eventually enough marks are made on the page that the subject has context. Clothes, background... and actions. At some point the scene depicted by this collection of lines and smudges becomes forbidden. What was an figment of someone's imagination is now a very real crime.
How does that happen, and when? Who specifically does this law protect? Is the person who drew it a criminal, or is it only a crime when someone buys it? Is every viewer of the picture a criminal or just the ones who enjoy it? How do you tell which is which? What about the imagination that spawned the picture? Would the artist have been a criminal if they hadn't put their mental image to paper? I find these questions very difficult to answer in a way that makes sense for a society. Every seemingly obvious answer can lead to some very harmful laws.
But the main motivation is one of greater public good. A scribble that harms nobody is made illegal because by locking up the people who like the scribbles, they cannot remain free to eventually harm real people in the same way. It's a noble cause and perhaps an effective law (I have not seen proof one way or the other). However it is also disturbingly close to pre-crime. I'm not entirely comfortable with that.