Legally speaking, the crime of conspiracy that he was charged with isn't anything new, nor is the manner in which he was convicted for it. If two or more people sat around talking about how they wanted to kidnap X person back in 1978, that could be enough to establish an agreement to commit a crime. In some jurisdictions, that alone is enough for a conspiracy conviction, although many jurisdictions also require that one of the conspirators make an "overt act" that is a step toward execution of the crime (such as Googling "how to kidnap a woman" or e-mailing details about how the kidnapping would take place, like this guy did).
You can think about anything horrific that you want to, and thinking it alone is not a crime. But if you speak about it with other people, and you speak about it in a way that can be deemed as an agreement to perpetrate a criminal act, then it becomes a conspiracy, which has been around in common law for centuries.