I submit to you that it is not a mistake, the failure is deliberate!

There is a method to the madness, and the method even has a name: the Cloward-Piven Strategy. It was first elucidated in the 1960s by a pair of radical leftist Columbia University professors, Richard Andrew Cloward and Frances Fox Piven:

The strategy of forcing political change through orchestrated crisis…. …the “Cloward-Piven Strategy” seeks to hasten the fall of capitalism by overloading the government bureaucracy with a flood of impossible demands, thus pushing society into crisis and economic collapse.

In hindsight, surely calling the police and having them personally remove the weapon from his property would have been better. Then, there's no OMG moment of the Chief's seeing a gun plop down on his desk. Yes, he's stuck in that he didn't know he was dealing with a firearm until he looked inside the plastic bag, at which point said firearm is technically in his possession and he's apparently in violation of the letter of a law. At some point, though, common sense has to take over. I'd say place the bag back out in the garden, call the cops, and let them retrieve it as you found it with the understanding that it is not yours and you want it off of your property.

Of course, it's easy to be an armchair quarterback in such a situation. If a firearm were to show up unattended on my property, who knows what my reflexes would lead me to do. As for the premise of feeling better about the USA, I wouldn't put it past some of our cities and their courts to make the same series of mistakes that has happened across the pond in this case.