This is a topic I've researched quite a bit and interviewed cops about.
To put it simply, saying a knife is not for defense when questioned is a very vague rule of thumb and not a universal piece of advice. Many variables can affect how you answer so it takes a judgement call.
In the following cases you should tell the truth about your knife if it's indeed for self-defense:
-The law in
your specific jurisdiction, including case law, clearly establishes the legality of your knife and leaves no room for interpretation based on intent.
-The knife is very obviously designed to be a weapon or carried in a manner suggesting weapon use (e.g. ankle sheath). The cop is going to know you're lying and just going to be more aggravated at you.
The reason for answering that it's not for self-defense when the knife could be considered utilitarian or when the law contains some sort of intent-based language has a bit to do with cop psychology and sociology. Police officers are a behavioral ingroup and though human, often develop observable patterns in their behavior and views of others as a consequence of their job. One of those patterns is called "collective narcissism," where the job goes to their head a bit and they not only feel morally superior, but consider their
legally-sanctioned ability to use violence to be a right only they have. Their job is to "protect and serve" and so a private citizen who shows they can "protect and serve" themselves is a perceived threat to their ego. This becomes apparent if you've ever watched police testimony in front of legislatures on the topics of handgun carry laws or knife laws.
A secondary reason is most people they deal with on a daily basis are legitimate criminals (people with warrants, criminal record, drug history, child abuses, partner battering), and it is all too common for these low-lifes to have a knife and to lamely make the excuse "it's for defense." This can cause a cop to be suspicious of
anyone saying a knife is for defense.
A third reason is that most cops I've talked to have specific opinions about knives as defense weapons in general. They are of the opinion that they are not good weapons for defense, and I have to admit they are not alone in that opinion. Knives have very little "stopping power," require you to get too close, and are considered deadly force. One cop called them "the schmuck's defense" because they are so often carried by people who have absolutely no training on how to carry or use a knife as a weapon and think they are going to be effective with it (e.g. those who carry a folder in their purse or backpack, or mall ninjas), with little appreciation for how the use of force continuum works that could get them in serious trouble even if they fend the attacker off (such as fatally stabbing an unarmed assailant).
In jurisdictions where "intent to use as a weapon" is a variable under the law, the above factors can swing that cop's judgement toward illegality if told it is for defense. Even in jurisdictions where there has to be intent for "unlawful" use, most people forget that stabbing people is always unlawful and it takes a court to decide to excuse that unlawful behavior on self-defense grounds.