"Like kind" analysis
I've seen General District Court judges rule that something with a sharp edge on it that's not a "scout knife" is of "like kind", without specifying "like what", with respect to the four kinds of prohibited edged weapons. There was one case, thankfully overturned on appeal, in which a box-cutter was declared "of like kind" to a straight-razor.
In addition to what P.N. said about dirks and bowies: a dirk is a stabbing weapon, double-edged, and of sufficient length to penetrate the vitals, e.g., at least ten inches. A bowie is the frontiersman's friend, good for skinning a bear as well as hand-to-hand combat. They generally have a dropped point and a swept blade with a pronounced curvature (the picture P.N. used is a milder curve than most I've seen) and sometimes a blood-letting groove. Minimum length is generally eight to twelve inches.
The other two are switchblades, spring-loaded with locking blades that are revealed by pressing a release catch, button, or switch, and ballistic knives, which are often cylindrical and generally have no "handle". They are contained in the bottom of a tube of some kind and are under tremendous spring tension. When a catch (similar to a switchblade), the entire knife is ejected from the tube. If you saw "The Punisher", there's a scene in which the protagonist is crawling out of his overturned car and his would-be assassin jokes about how he brought a knife to a gunfight. There's a click, and the ballistic knife is sticking through the guy's throat, which is leaking profusely, to the accompaniament of his gurgling noises.
What people are really afraid of with knives is the "silent killer" thing (other than after I've eaten too much garlic, that is, though they should know enough to fear that as well). Same reason silencers are restricted. Guns make a lot of noise, and it's hard to kill someone with a gun and not alert someone to the situation. The knives in those four categories (and of course you can add shuriken, nunchucks, machetes, and straight-razors to the list) are regarded as murderers' weapons. I think it's really that simple.
My favorite for carrying around in the woods is a Fallkniven S1, picture attached. Because it's a single edged knife with a slightly swept blade and dropped point, I have no doubt that a local court would characterize my camping/fishing knife as a "bowie", even though it's about four and a half inches long.
