Why would you have to justify owning it. No different than anything else.
I would think its intended use would be to stick something with it and bring a quicker conclusion to the encounter. In reality, its probably to awkward to use to be practical. Then again, it might be just the ticket for certain applications that dont swim away.
These days, its probably smart just to get in before the ban. Like most other things of this type, once word gets out, the do gooders are soon on it, and they all of a sudden become instant collectors. $400 will soon be cheap.
I had one of the (supposed) "Spetznatz" blade shooting knives a number of years back. Paid next to nothing for it new, and traded it off a few years later for a substantial gain (of course by then it had been "banned"). Even then, I got the better end of the trade too. The thing was junk.