Willy,
He wasn't prosecuted because he was in posession of child pornography.
That is what you are missing here. Yes, child porn is illegal - I don't debate that.
He was prosecuted because he wrote down a story depicting a crime that he may or may not commit. In this case the subject was child porn, but the ability of the prosecution to convict wasn't based on the subject matter,but the fact that he wrote it. Now that the precident is set, any one else can be convicted of similar crimes and sent to prison for writing a story.
Again - to make my point - the subject matter isn't important in this case, but the fact that the accused put his thoughts on paper.
Example:
Arson is a crime - NOW so is writing about commiting arson.
Mr. Daltons' crime was NOT posession of child porn.
He wrota a story and went to prison for it.