Cops are just people, they are supposed to know the law well to enforce it, but they are bound to make mistakes, we all do. All they really need is a *reasonable suspicion that you are doing something wrong to get involved.
Cop probably knew, hey no flying things in park, he's flying something. Came on over. Droner opted to run his mouth, tell the cop what the law is, and escalated the situation. We only know parts of the story, as the droner cut various parts of the actual incident. I doubt they both just sat their in silence while several minutes went by.
The cop is not there to argue, or interpret that law. He just enforces it. Judges interpret law. Literally arguing with a cop over the specific wording of the law/rule is not something they will do. Thus, he got a ticket. Cop moves on with life, forgets all about it.
Droner was looking for a fight, he got one. Spend a lot of time and money. Could everyone have gotten what they wanted, and the cops learnt a lesson without all that time, money, and handcuffs? Definitely.