How did this movie lead to banning nunchakus?

It was actually Enter the Dragon that led to the banning of nunchakku in Britain. The chairman of the board of film censors was driving around after the film had been released in Britain and he seen kids in the streets using home made nunchakku and reported this to the rest of the board.

It was the paranoia and ignorance of the left-wing, ultra-liberal California legislature, liberals who do not believe in the concept of self-defense, believing that it only adds to the violence. The law-abiding citizen is supposed to leave self-defense to the police. Somehow, I hold martials artists responsible also. During the Bruce Lee movie era, as the story goes, some self-styled martial artists, perhaps seeking to impress, told the California legislators that the nunchaku could generate 1600-lbs of impact pressure per square inch. The nunchaku was subsequently the ultimate weapon of mass destruction. Suitably impressed and frightened, the ignorant legislators promptly banned the nunchaku, not realizing how dangerous the nunchaku was to the OPERATOR, more than to its target.