I suppose it's because "butter knife" has the word "knife" in it.
A few years ago, on another platform, I bought a sheath for my swiss army knife. The knife wasn't included, only the sheath was for sale. The sale was restricted to adults only and I had to prove my age to the delivery driver. I expect I couldn't have bought to start with without having a pre-registered credit card = I was over 18 years old because I had a credit card. I suspect it was because the product name included the word "knife".
Some years ago, on another platform, I bought a set of breadmaking accessories that included a "lame" (e with an acute accent). In case you don't already know, it's an extremely sharp thin blade for slicing plain or decorative cuts into bread dough. The open design and extreme sharpness meant it could be used as a lethal weapon. Came with a very necessary sheath. No age restrictions or any restrictions on buying it. Scores of lames available, without any restriction, as I browsed a lot before buying one. The only "safety" feature of a lame is that someone trying to use it as a weapon is likely to cut themselves.
Oddly, I've bought a couple of 'craft knives' recently, elsewhere, and faced no age restrictions. Such knives are really sharp, designed to cut through thick cardstock, vinyl, silicone, set resin... However, modern ones are designed to be as safe as poss, including one model I have which is designed so you don't touch any part of the blade (not even the spare blades as a mechanism swaps in a new blade without exposing any of it) unless you specifically decide to touch the end of it when using it. But, like "butter knife" the word "knife" forms part of the description.
Knife rules seem inconsistent!
