Hmmmmm. Yes, technically they could but IMO, not without a genuine reason to suspect there is a safety concern.
Eg; if there is a blown headlight, that would be the opportunity to have a "safety/roadworthy" inspection.
I find that completely unreasonable to be charged for possessing a fishing knife, particularly in the presence of fishing rods.
It potentially sounds like the cops got a bit of attitude and it was payback time (not casting aspersions BTW). It's just what I've experienced in my younger days from the Rozzas