Hi Debster,
Thanks for the message. Let's talk about this concept, because it is an important one.
First, what you are seeing in the Restrictions section is the start of a concept of that is extremely important in Logic Games, and one that will be referenced repeatedly through the book. Thus, I'm glad it is helping so far!
Second, nothing is really mandatory in diagramming
However, I think you will find that in the long run, diagramming Not Laws is extremely helpful, especially as the game gets more complex. Not Laws allow you to easily control the information in a visual fashion, and they also indicate absolutes in the game, which play a key role in many questions. However, I always tell students that they have to approach games in the way that is most comfortable with them. If that means not diagramming Not Laws, then that's ok (but give it some time before making that decision; you are still pretty early on in the book, and a lot of the ideas we talk about early on are the foundation for strategies and approaches we introduce later).
Please let me know if that helps. Thanks!