Music in the Movie?

Not really out of place. For a long time, The Ventures were "number one in Japan".
Japanese kids liked the same music that American and European kids liked.
There's a time capsule element to Imamura's films. Without making a fuss about it, there's a documentary backround to his films showing the means of transport, food, fashion,music and politcal protest. Many films do this of course. IMO, he's just more subtle.

I think that scene where the wife is flipping out in the assylum that morphs into
her screaming behind bars on pavement in the middle of the sea (to the tune of surf music) has got to be the first rock video.

Excuse me. I just got out of the shower.
Le Cercle Rouge

The fact is that "surf rock" originated in US, but (luckily) was "imposed" on many other different countries and cultures...Japan was literally invaded by american popular music, indeed generating a fair amount of - for this particular case - surf-rock bands (and Japanese are masters for "cover" bands).
As an example, Italian movies from the same period (the 60s) are full of surf music: it was the fashion at that time...not to mention how surf music was popular in South Vietnam during the war.
Probably one expects "serious" or traditional film scores from "exotic" countries, but it's a bit too ethnocentric conception...don't you think?