So he basically got arrested for making toy guns.
Yes.
It is easy to see the philisophical reasons for the law.
It serves two purposes for those who demand a disarmed populaition.
First, it reinforces the concept that guns are bad and must be tightly controlled by the state. Even facimilies are bad because they promote the idea of guns.
Second, it removes any restraint on searches for "guns" by the police. If anything like a gun is seen, it is reason for a search, because possession of even a facsimile of a gun is illegal without a special license.
All of this is designed to delegitimize the idea of ownership of guns.