Hi Mark!
I´ve asked one of our forum members (Tad, who is from Japan) about why soul music seems to be so popular in Japan, don´t really remember now exactly what he answered, but don´t think he had any one explanation as to why rare soul seems to be so popular over there. But he agreed that whenever you search the net for some very rare and elusive soul cut, you´re bound to end up on a page put up by a Japanese soul enthusiast! And whenever you hear that "someone just visited the store and bought 10 copies of the same album", well, that´s often a Japanese guy who has come to your country to buy some items that may be a bit easier to find in your country, but very rare in Japan. I remember once I sold a European-only Gwen McCrae album to a Japanese person, who was willing to pay any sum of money in order to get it. Anyway, as a whole I guess the Japanese are into soul music for more or less the same reason(s) as the British/European punters, whatever those reasons may be...
Yes, the Jappies own the rights to some of the very obscure stuff they put out, but there are also quite a number of bootlegs and cases where no mastertapes are used, instead copies being made of vinyl records that are digitalised.
Maybe not of much help, but at least I tried
Best,
Melismo