I've always said this. If a knife is a weapon it falls under the 2nd as a form of arms. If it isn't a weapon, there's no need to ban it. Simple.
Anything can be used as a weapon in the extreme, and most things can be weapons in typical situations, but knives are kinda like shovels or hammers. They're a tool that can be a weapon easily. Even knives designed to be weapons from the start (and there are plenty) are such limited weapons, only idiots would ban them outright. Well, that might be a bit strong, but it would require ignorance of knives and their use as weapons.