the US basically wrote all censorship laws in Japan from the occupation in the 1950s as we rewrote their constitution to be more like a mini-america. this included the then current censorship. in the 1960s, the US had a movement called "the sexual revolution". this did not happen in Japan (which has a negative birth rate, btw). they more or less have the exact same censorship laws as 1950s america did, and with only minor updates over the years. so when mainstream porn was officially legalized by a scotus verdict(IIRC, it involved the film "deepthroat" among others) that ruled that it is a legal distinction between actors depicting sexual acts and not people merely hired to perform said acts(prostitution), and classified it thinly as art under the first amendment, this also did not happen in Japan. which is why mosiacs and tentacles/phallic substitutes are somewhat popular in Japan.