I'll Chime in here for fun. I imagine it is illegal to carry certain knives because they are designed to stab or quickly deploy into a combat or offensive posture and that is basically their main design intent. They are not really for cutting fruit or chopping wood etc. Not that they can't do those things but that is not their design intent. whereas, a screwdriver or a hammer's design intent is for what it obviously is, construction or repairs etc. Not that you can't stab or injure someone gravely with a work tool, but it's main design is not for that and we need tools just like we need knives that are designed to butter bread or cut steaks.
Now having said that, carrying a designated "weapon" knife is illegal because certain "types" meaning criminally inclined types, I'll let you interpret that as you will, are more likely to carry these types of "weapons" and this gives the police a tool to get those inclined to use a "weapon" illegally off the streets before they actually use the so called "weapon" illegally as a weapon.
Lastly, it is still illegal to stab or smash someone with a hammer or screwdriver, so if one of these so called tools is used as a weapon, there are specific laws that cover this intent or actual illegal use as a weapon. Also, this only would apply if the attack were done unprovoked and not in self defense.
Also, it might be worth mentioning that police officers are allowed some discretion in how they apply the law and to whom they apply it. If you are say a UPS drier or someone who daily and frequently in the course of your work uses a knife legitimately to cut things and were stopped going home with say a 4 or 5 inch knife and had a good attitude, I think many officers would just warn you that it was illegal and let you go without taking any action. But if you were tattooed with prison tattoos and riding a Fat Boy and had a very bad attitude, I guess you would not be surprised if the police officer took your knife and you to jail for the night.
It gets complicated and it is right that there should be laws that protect us from people with ill intent before the intent becomes an attack against someone in a robbery or for no reason. Most cops, carry knives and completely understand that guys and knives go together like kids and puppies.
