Knife laws in the US are absurd. The leeway given to the police in interpreting the laws which are often vaguely written at best is beyond absurd and clearly open to abuse.
When the police are given a tool which they can abuse they often do so and generally speaking they tend to mostly use the vagueness to harass people from groups they dislike, be they bikers, hippie or people of color.
From The Truth About Knives: http://www.thetruthaboutknives.com/2016/03/this-is-why-you-cant-trust-knife-specs-on-the-internet/
March 15, 2016
What to do if you are a law-abiding citizen and want to make sure you are following the law on blade length? I can tell you one thing, measurements listed on websites are the last thing you should rely on. To wit:
The Ontario RAT-1 pictured above is listed as having a 3.5 inch blade on Amazon’s website, but based on my own measurements, whole length of exposed steel is 3.615 inches, while the sharpened edge is only 3.3625 inches. This may not sound like a big deal, but if you need your knife to be a certain length for legal reasons, this can be a headache waiting to happen.
The recent case where Amazon (UK) has “come under fire” for the specs listed on their site further highlights the issue.
From theguardian.com: “Amazon condemned for ‘cavalier’ knife advertising”
In the trial of a 16-year-old Aberdeen boy who stabbed and killed Bailey Gwynne, the court heard he had chosen the knife because “it said on Amazon ‘legal in the UK’ because the blade was under three inches”.
In fact the knife had a 3.25in (8.25cm) blade, making it illegal to carry in public…
In the UK, as I understand, it is illegal for someone under 18 to carry any knife, so this kid would have been in trouble regardless. What would have happened though, if one of the adult citizenry had purchased this particular knife, thinking it would be legal to carry?
This why blade lengths listed on websites can’t be trusted.
Part (all?) of the confusion certainly stems from how blade length is measured. You would think it would be pretty straightforward, but that is far from the case. Some states define this term (sometimes it is sharpened length, sometimes it is a tip-to-scale measurement), but most do not, and this is where you can find yourself in hot water. Sellers rely on the manufacturer to supply the specs and with no unifying definition, there is no way of knowing what that measurement means, or if two different knives from differing manufacturers are being measured by the same standard.
As an aside, Rhode Island law illustrates just how difficult it can be to properly define blade length. Their statute reads thusly (edited to the relevant text):