I did a search very quickly, apparently that's why the JAVs are censored, completely lunar the reason is
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Why is Japanese pornography still censored?
It comes down to current interpretation of Article 175 of the Criminal Code of Japan, which forbids distribution of “indecent” materials. So while there is no specific law requiring censorship, that’s the interpretation.
In 1957, a precedent was set based on a Japanese translation of DH Lawrence’s Lady Chatterley’s Lover. At the time, the Japanese Supreme Court ruled that obscenity was defined as “unnecessarily sexually stimulating, which damages the normal sexual sense of shame of ordinary people, or is against good sexual moral principles.”
Current interpretation of that ruling apparently means genitalia is obscene, but all the other stuff you see in Japanese porn is not. Make of that what you will.
As others have noted though, standards have changed over the years. While it was once considered indecent to show any pubic hair at all, that’s obviously changed.
There have been very few arrests for violations of the law, though. But there have been some convictions, which means Japan does have legal precedents upholding genitalia as indecent. In 2004, Yuji Suwa, the artist of the hentai manga series, Misshitsu, was brought to trial for violation of Article 175. He pled guilty and avoided jail time, but was fined ¥500,000. He tried to appeal to the Japanese Supreme Court using the argument that more indecent material could be found on the Internet and that Article 175 violated the Japanese Constitution’s protection of freedom of expression, but the Court didn’t listen. Instead, they increased the fine to ¥1.5 million.
Then in 2013, three people were arrested for selling obscene images with insufficient censoring. Since Suwa’s case and this 2013 case, many artists, publishers, and film studios have been more careful about self-censorship to avoid obscenity charges and after the Suwa verdict, some bookstores took down their adults-only section.
The law may eventually change in the future, but no one wants to be the one to run the risk associated with setting that precedent.