Knife laws
A knife is one of the most basic tools the world has ever seen, and by far one of the most useful. In the society we live in today knives are hardly as important (in a scalar sense) as they once were, but that's not to say that knives don't retain incalculable value as tools. When you need a knife, you need one
bad.
Now, despite the hoplophobia and ignorance of most suburbanites and city dwellers, the majority of knives are not intended to be weapons. They are tools first and foremost, though the fact that they are designed as tools does not preclude them from being used as weapons. How many tools do you have that would make highly effective weapons, eh? Hammer, crowbar, screwdrivers, length of chain, washers (put them in a sock...); I could go on for quite a while. How about an axe or hatchet? A simple two and a half foot 1" dowel makes a great club.
So why are knives singled out? This is a less easily answered question than that of the war on guns, you see, because guns are indisputably designed to kill; the argument there is more what kind of killing is acceptable (self defense, resistance of tyranny). And I don't have an answer for you, other than the fact that the government is striving to control the lives of the masses and to elevate the police and military (automatic knives, anyone?) from the plebeians.