Though with 20/20 hindsight, his comments about there not being a threat were wrong, I thinks its unfair to say they were "stupid".
In the end, Law Enforcement must make decisions every day regarding efficient use of resources. These decisions must be made by humans with out crystal balls and magic wands.
The Sheriff seemed to have based his decision on: A lack of violent history regarding the individuals. Lack of any evidence of truly threatening behavior.
I live in an ubber large city. There are alot of police, but there are probably hundreds (if not more) custody exchanges being made every week between parents involved in nasty divorces. One of those people making such an exchange is a family friend.
Yet..... can law enforcement be expected to appear at her exchange and hundreds of others like her based on: Divorce went south, ex is unpleasant, ex is non cooperative, ex pursues his legal interests relentlessly, He might...?
In a perfect world, the answer would be "yes". In the real world however.....