Because Ichimasa Sagawa his father was very rich and made bail for him, plus people went through the back door. So Sagawa Kazumasa went back to a mental hospital in Japan, and after 15 months of treatment, the head of the mental hospital declared that he had been miraculously cured, and would not commit any further socially detrimental acts at all. Another reason is that he killed a Dutchman, and the Dutch government is no longer pursuing him.
And Koza's perverted domain also reflects the traditional arrogance of the Japanese nation, and this arrogance and perversion was also very popular with Japanese society at that time. Ichimasa Sagawa went on to do very well, publishing four novels and a book of poems, working as a columnist for a famous Japanese gourmet magazine, and making guest appearances in TV dramas and movies. After treatment in a psychiatric hospital, he walks the streets of Japan as a free and successful man, as if he had never done anything to hurt anyone. Yet he is the only cannibal in the world who has been found out and still gets away with it. Most people still can't accept the existence of such a person, but again, I don't know why he wasn't sentenced to death, some say it was because France had just abolished the death penalty at the time, so he wasn't sentenced to death, and that at the time he was mentally ill.
After returning to Japan, he entered a psychiatric hospital, and after a long period of observation, the Japanese police thought that he did not commit the crime because of mental problems, but because of personality defects, and was originally prosecuted, but France rejected the prosecution. So he was not sentenced. By the end of the day everyone was just forgetting about it. He didn't commit murder again. This also shows that he personally has no mental problems per se, but instead the Japanese media creates a lot of perverted articles for some perverted Japanese to worship.
No matter what the reason was, he deserved to be punished for committing acts that harmed everyone, but he eventually changed himself and didn't commit any more acts that were harmful to society, so as long as he could change for the better it was forgivable.