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.
I think it has to do with "lazy" pronunciation merging them over time. The kana for the particle "wa" is also normally pronounced as "ha," the kana for the particle "e" is normally prounced "he," and so on.