why is japense porn censored?

Are you asking why porn is censored in Japan?

If so, it's due to Article 175 of the Criminal Code, which bans public display of obscene content, conventionally interpreted to include genitalia. This leads to self-censorship in the industry, where creators use mosaics to comply with the law, despite Japan's constitutional guarantee of free expression.

Because it's against the religion or something like that the also aren't aloud to watch media with decapitation