To disallow any discretion on the part of individual officers is to usher in a police state. I know an Arizona cop who pulled over a guy who was speeding. He was fully cooperative and harmless, but they asked him if they could search his car and he, feeling nothing to hide, answered yes. So they searched.
Under the passenger's seat, all caked in dust and debris, was a Jennings J22 pistol. No ammunition was in the car and there was no magazine. It looked like it had been kicking around the car for some time. The driver said he had no idea where it came from and these two cops believed him.
But they still arrested him.
Cops, magistrates, juries, judges and what not have, and are supposed to have discretion, otherwise we end up with a police state.
With knives, 4-inchers can be adequately used as a weapon and longer blades really don't add much to their lethality. Police have to look at the guy they've stopped and size him up. Take away discretion and it doesn't work.
Perception is everything. Those piece of junk Jennings pistols are often seen as a weapon of choice for thugs due to their low cost. I'm not so sure that this guy would have been arrested if the cops found an expensive, high quality handgun. It's not fair but this is exactly why discretion in necessary.