Seriously, if you're a celeb over there and caught smoking a joint, your career is basically over.
Even the infamously prudish U.S. is more accepting of it.
I'm guessing it's a leftover of U.S, imperialism, like how all their porn is still censored.
A lot of it is leftover from the modernization period, when the Japanese ruling class intentionally affected as many European mannerisms and attitudes as they could.
But there was a second wave of cracking down on degeneracy when they went fascist. Much like West Germany, the US put a lot of the fascists back into power when they took over after the war - and it's not like MacArthur was interested in pushing back against it.