I lived and played in Japan for close to 20 years, and I remember that the Japan Open had a funny rule in place where amateurs couldn't receive the full payout. I was told it had to do with the way Japanese gambling laws were set up. Same for any other pro sports event in Japan where amateurs could compete. Golf, tennis, bowling, ect.
Always thought the gambling and prostitution laws there were extremely conservative on the one hand, yet extremely easy to circumvent on the other. Examples I can think of were pachinko parlors completely legal because money winnings aren't exchanged on the premises. You exchange your balls or coins for token 'gifts' which are then technically 'sold' or exchanged for hard cash in the parking lot or down the street.
The yoshiwara red light or soapland district is completely legal according to Japanese prostitution laws because the customer is paying for the service of being bathed by a sub contracted woman, and whatever happens in the room between the male client and the sub contracted woman is not described in the service menus, and is a private matter between two consenting adults.
That goofy gambling law is exactly why I absolutely refused to win the Japan Open!..................Three times!
Dave