You're correct that it's knee-jerk public perception, but you're wrong on the time, the US Switchblade Act of 1958 was a reaction to teen gang movies of the '50s. Similarly, states enacted laws covering balisongs (California is especially tough) due to public perception that Asian gangs were running amok with them. Knife laws are often poorly defined and intentionally vague so that police and prosecutors have more flexibility in how they choose to enforce them.