The question should always remain "why arrest" and I submit anyone who is unsure why that is should, themselves, suffer arrest and imprisonment for no less than 3 months to help answer their question.
Sometimes law enforcement has to choose to not enforce a law, so “why not arrest” is just as important of a question as “why arrest” when choosing to enforce a law.
If it reaches a point of causing public disorder or harming the general welfare of a democratic society, then why not arrest?