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Girls Delta = softcore only and no VR. Great quality, but extremely limited rotating library selection and last time I checked, you could not even save individual images, but only download entire photo-sets.
Nyoshin.com = hodgepodge collection of fetish movies that most probably are not interested in. Updates appear to be recycled content.
JapanHDV = Pretty much (if not entirely) content licensed from Dreamroom originally appearing on sites like Caribbeancom, 1Pondo, etc. They now have someone shooting exclusive for them at Tenshigao (meaning Angel Face).
FC2 = massive collection of amateur-shot content. Very few diamonds among a gargantuan amount of extremely poor content.
What these sites all have in common, as Apollo noted, is they are not based nor incorporated in Japan. Production, however, is most likely in Japan (it's beyond obvious at times). It isn't illegal to shoot UC in Japan (because all porn starts that way). The illegality comes from distributing within the country.
The question asked a lot is then, "why not hire <big name actress> and shoot her outside of the country?". The simple answer is a combination of contractual and legality issues prevent this from being a reality. Those who get caught shooting UC in Japan even when it's distributed via abroad sources but are still very much targeting users in Japan can have a rough experience with the police.
Edit: in addition for FC2 at least, even if you find good content, it is EXTREMELY difficult to get in touch with the director/seller. That FC2 seems to be the main target of UC "busts" in Japan means those who sell there more often than not keep a very low profile. Sometimes content may sell at a premium ($100 a scene) and with an actual limit (20 buyers only and then it is removed) in order to remain clandestine. Thus, from my experience at least, even if you find glorious uncensored content, the chances of actually getting in touch with the seller is close to nil.