A really simple set of issues here is overlooked when talk about "copying" comes up -- knives, like other tools and mechanisms, are generally considered under the realm of patents and not copyrights. For this reason, knife designs become public domain much more quickly than movies or songs.
Besides, to really address this, we should also be discussing things like the original bowie design (whatever that may be) being co-opted by all sorts of makers and manufacturers, or liner locks to be a bit more contemporary, or even the American tanto blade (to be a bit more on point -- excuse the pun -- relating to Cold Steel).
While we love our knives & tend to think of the artistic merits of their designs, we also love them for their utility, & it's exactly that utility that brings them into the realm of patents, not copyrights.
There might be some exceptions, but they would not be the general rule. And these issues, of course, were originally addressed in the U.S. Constitution.