That's always confused the crap out of me, oh and don't get me started on Japan actually censoring porn... Just... Why?
Japan is... unusual. Keep in mind it is the same culture that produced this:
As I understand, that's actually the
result of Japan's censorship of porn: apparently, depictions of erect penises were (are?) obscene, but tentacles were not... And so a fetish is born.
That woodcut is much older than that...