One explanation that I once saw is that the US got them made in Japanicon for the Korean War because they were short of them. I think that this explanation was the dealer trying to bump up the price several times because he had one for sale.

I found the one that I have at a car boot sale about 30 years ago at the bottom of a box of military clothing/belts etc. I asked the woman running the stall how much the bayonet was and she was like "Oh my god it's a bayonet." I replied that I didn't mind and she said that I could have it for £2.