I know it may sound crazy, but not everyone is driving around with a phone. That's why you sometimes see updatable signs on highways telling you what AM station to tune into for whatever alert it is they have to issue .
You're right that AM is mostly used by right-wing hatemongers in the US, but they will always find something else. It's not the platform that's the problem.
It's a bit funny to me that this move is coming from Japan. It's been a little more than a decade now, but I used to live in Japan, and good radio stations playing music were all over AM radio when I was there. Maybe it's regional.
Never heard of it.