Usage. Design intent. The law. Perception.
Usage- Many things can be used as weapons, even if they weren't designed as weapons, or if they are actually impractical for that purpose.
Design intent- Some knives are specifically designed as weapons, some are specifically not designed as weapons. No one is going to tell me a Gerber Mark2 or a Fairbairn/Sykes dagger aren't weapons, or that they weren't designed as weapons.
The law- If the law says your knife is an illegal "weapon", or that you have used a knife unlawfully as a "weapon", and if a prosecutor can convince a jury to agree, it's not going to matter if YOU don't consider the knife to be a weapon, or what the intent of the designer was, you're doing time.
Perception- Every individual has their own perception of knives in general, and individual knives specifically. Some people truly look upon a SAK as a "weapon". A cop may look upon any knife that could harm them as a "weapon", or a "potential weapon", regardless of it's design.
Most people who are serious about carrying a SD knife would not use the SD knife for utility tasks and dull it's edge. They would carry 2 blades, one as a beater and for actual utilitarian use, and their SD knife which they keep razor sharp with a keen edge.
I disagree with this. I carried the knife below for several years. I used it at work (shipping, construction). If the blade started dulling from use it was easy enough to pull out a pocket hone and touch up the edge (did this for work). And as I was advised by my attorney, I left tape residue on the blade, and small amounts of drywall mud here an there in the handle so that if a prosecutor tried to argue that I was carrying the knife specifically as a weapon, to hurt people, my attorney could argue that evidence clearly showed that I used the knife for work. A knife doesn't have to be pristine to be used effectively as a weapon.
Anyone who is serious about self-defense considers the possibility that they might end up in court.
On the subject of cops, I used to openly carry fixed-blades wherever I went. Did so for many years here in San Diego. I'm a biker, I look the part, several cops have seen me carrying a fixed-blade and never said a word about it or stopped me for questioning. Even on those occasions when I've been stopped by the cops (traffic/vehicle code violation) they've never given me a hard time about my knife (or anything else).
My last encounter with cops while openly carrying a fixed-blade went like this- Cops stopped me for an improperly mounted license plate. Lead cop asked me why I was carrying the knife. I said "To cut stuff". Cop asked me if I had any other weapons on me. I said "No. But I do have 2 pocket knives on me". He had no interest in those, didn't have me hand them over or even ask where they were. He gave me a friendly warning about my license plate, told me to adjust it when I got home, and sent me on my way with all my knives, without a ticket, and without any attitude or hassle.