This is likely a good thing about the way the law is actually worded. While the police may sometimes use terms like "legitimate purpose" with respect to knives, the law simply says you can't carry one with the intention of using it as a weapon.
There's a problem there too if that what the law actually says. (I'm taking your word for it.)
What's wrong with carrying a knife with the intent to use it for self-defense, and therefore as a weapon? There's no problem in my book. Heck, I'm in favor of concealed carry of firearms, which would be a weapon.
If that's what the law says, then everybody has to do this fake dance about carrying their knife as a tool only. For example, "I'm a mover officer, and I open boxes all day; please don't arrest me; I promise I can show you my business card here."
It's not just a suggested fake dance. The person actually has to plan out their fake routine meticulously so they don't get arrested, charged, and convicted, merely because they're carrying a knife.
I see red flags when a knife law restricts behavior within the law. If the officer is not satisfied with what you're saying, then the officer can go ahead and make an arrest because he can. Getting arrested is a potential life changer, depending on your job and lifestyle.
Make a separate law directed toward behavior (e.g., threatening to kill someone is a crime). Really, what's the problem there?