unfortunately in Toronto, Canada there is no law...it is left up to the officer to make a call and for the officer to defend his decision in court, should it come to that. i couldnt really justify using this method, i only point out that other officers of the law do use it and that the trend of what officers will use as a benchmark will vary greatly from area to area...i support people carrying knives safely and legally, my cousin believes that carrying a knife should be illegal without a permit. we are both willing to arrest, confiscate and then defend our decisions in court.
i agree with your point that unknown laws are arbitrary and inherently unfair, but i must point out that it is the duty of the citizen to research these laws, and the cops to only enforce them.
it is mostly (aside from a few exceptions) up to the officers discretion - that is the law (Toronto, ON)
this policy IMHO has been left in place to allow officers who work in tougher neighborhoods where knives under 4 inches are common place in stabbings to arrest detain and question those who seek to ruin EDC knives for the rest of us. It also allows those who need big ass bowie knives to continue to carry those....in some states (US) its automatic confiscation.