Usually they ask what I'm doing with the knife, or something along the lines of that
usually ill answer one of two things, if the cop is being a dick, ill usually say something like
"Well officer was mugged a few years ago, and since you guys never showed, and even though I've gone through all the legal loopholes you guys wont grant me my license to carry permit."
If the cops being a cool guy like they usually are I'll just tell them because its within my legal right, and that although i know they are here to protect me, there isn't enough of them to go around, therefore I've taken it upon myself to defend myself, after all the police aren't there to prevent crime, they're there to clean up after it."
I see. I am surprised that in either case you openly tell them it is for self-defense. It has been my observation that for whatever reason police don't like that answer. I was listening to this younger kid who was also from CA who was complaining that the police took away his crowbar (no ticket or arrest in this case) because he answered that it was for protection against dogs. This kid had been bitten the previous week by a pitbull. But they took his crowbar anyway because it showed intent to use it as a weapon.
I suppose (this is just me speculating now) one reason they let you go is unlike crowbars ("club" weapon under CA law), an open-carried fixed blade is clearly established as legal per statute. This is as opposed to other laws and jurisdictions where the specific statute in question carries some kind of "intent" language. Either way, the police are obligated to investigate because it's their job to look into things that may not be crimes, but possibly could be cause for concern. They don't usually prevent crime, but they can try.