What it boils down to is the illusion of safety.
Exactly. The illusion of safety is the reason for most, if not all, silly laws.
Speed limits are a favourite of mine in that respect. Research seems to indicate that there is no difference in the amount of accidents on motorways between, say Germany, which has no speed limit, and the UK, which has a very strict one. The reason for this is obvious: The speed limit does not deter the reckless drivers, nor is it speed itself that causes accidents. But many people buy into it. So speed limits don´t make the roads safer.
Weapon laws, however, go one step further. Not only do they not make the streets safer, they actually make them more dangerous. Imagine you´re a crook in an area where most people carry knives that can be drawn quickly and a lot of them also hav a gun. Would you feel inclined to rob a random person on the street, knowing that there´s a good chance they´d fight back with lethal weapons? Of course not.
But what if you knew that 99,9% of people are unarmed, whereas YOU, the criminal, have a weapon? You´d go for it, safe in the knowledge that there is very little risk of you getting injured or killed by your intended victim.
Now silly laws, like speed limits, I can live with. At least they don´t hurt me, it just means I lose a couple of minutes of my time. But not being able to defend myself... that´s just wrong.
I have never, in my entire life, been in a situation where I needed a weapon to defend myself, and I sure hope it stays that way. I don´t go into dangerous neighbourhoods, stay away from seedy areas etc, but if push comes to shove, I will ignore the law, just like the person attacking me.
As for the movie influence, it´s huge. Even though people know that it´s just a movie, they still believe a lot of what they see on the screen to be true. They will see people fire shotguns one-handed (or better, in Predator, a minigun) and no matter how often you tell them that this would only lead to a sprained wrist or worse, they will always think of that scene when they talk about shotguns. And it´s not because they´re stupid, it´s because it´s the only experience they have of a shotgun.
Let´s hope that "I am Legend" (a god-awful movie, sadly) will make people understand the reason why automatic knives were invented in the first place.