Watsuki's case was a little different. He did have a large collection of old child porn, but it was legally purchased back in the day. In Japan it was legal for teens age thirteen and up to star in pornography. Their child porn laws were only recently reformed to match the global standard. Watsuki wasn't trading pics of small children with other pervs, he bought his entire collection at a department store on VHS. He got caught with it when he sought help getting it transferred to DVD rom, since the tapes were so old. He claims he legitimately didn't realize the laws had changed. Possessing that old stuff still isn't illegal in Japan, depending on the area you live in, but transferring its medium is, which is what he got charged with.
Not saying this to excuse his behavior, but it was a very different environment both culturally and legally.