Japanese works do not spontaneously manifest in other markets: they are consciously stolen. Someone purchases an original, ignores the "do not copy this" printed all over it, and makes a digital or hard copy which is then released overseas or to the internet without permission or legal right... therefore breaking JAPANESE law (and likely a number of international trade agreements in the process).
Someone MUST obtain an original in the country of origin (through legal or illegal means) before it can begin circulating elsewhere. The material itself has still been stolen and is in violation of the original creator/owner's copyright... reguardless of how far down the line you come across it.
Paying for a bootleg does not make that bootleg "legal". And while the PURCHASE of that bootleg may not be an illegal act in itself: the material you've purchased is still an illegal copy.
By the time it gets to you, it may no longer be "prosecutable", but it is no more legitimate.
I could give a **** if someone wants to download something they haven't paid for. I do it all the time. But I would like to think we make those decisions conscious of their implications, legality and relative morality. If you're cool with that: great. But I don't much like the argument that "unliscenced works are legal." Because it's not true at all, in any sense of the word. You can argue that DOWNLOADING them isn't explicitly ILLEGAL... you can argue that they indirectly benefit the industry and fandom immensely... you can argue that they don't hurt anyone (probably with less success)... but you can't find ANYTHING to prove that they are "legal" or legitimate in any way. Because they aren't.