In my state, Florida, everything except a firearm and a "common pocket knife" is considered a weapon. If you're wondering, the laws dealing with firearms are specific to firearms, hence the difference in definitions, plus Florida is a pretty cool state with very reasonable carry and self-defense laws, that understands not every gun is for killing people, and concealed carry permits are easy for law abiding people to get.
Anyway, I think this is because the other knife laws are old and just stuck on the books; just like how I'm not allowed to bathe inside(yes, that's INSIDE) my house or sleep with an aligator.
"Common pocket knife" acknowledges that these are just ordinary tools. I have been arested and taken to jail for battery(really mutual combat, but it's all battery, no not against women) with a knife in my pocket. Since I didn't use the knife in the fight, it was just taken along with my clothes and boots and wallet and other gear I had on me and was returned with them upon my release.