In my neck of the woods, it is a product of years and years of indoctrination by media, the ruling politicians, and schools. People are taught to fear knives as weapons, and view people who carry one with suspicion. It's really ironic that this is coming from people who use a large chef's knife at home every day and think nothing of it. I have a coworker who has reported me to my superiors for having weapons at work, yet, when there is a package to open, this same person wants me to open it.
Personally, I take it as an insult when someone views me that way when they see a knife on me. I feel that they are saying that they believe that I am capable of mayhem. They are basically pessimistic in nature, and always assume the worst possible explanation for any given situation. And, if the truth were told, I honestly believe that with a lot of these people, you are actually getting a glimpse into their own hearts. They think in terms of violence, and project that self image on others. How's that from someone who views psychology as a pseudo-science?