From reading the article, the crucial point is that Dixon's clients told him they were going to use his training to do illegal stuff. Up to that point, Dixon's teachings were protected by the First Amendment. He should have told them to get lost and reported them to the appropriate authorities.

E.g., it's perfectly legal to write articles and teach classes and post YouTube videos on picking locks. Even if the knowledge you spread causes lock makers to lose money and their employees to lose jobs. Indeed, your teachings help society by exposing false security.

However, if someone tells you they are planning a burglary spree and tries to engage you to give a seminar on unusual locks, then you better tell them to take a hike, or you are prosecutable.