This was something I really noticed coming back to Japan after two years in Korea – suddenly whisky highballs were everywhere, including in a bizarre mix with takoyaki in a chain of casual snack bars.
You can find lots of comment on this by Googling highball in Japan, but something no one seems to have mentioned is that the rise of highball was almost exactly the same time as the Japanese government plugging the tax loophole that allowed first happoshu (low malt beer-like drinks) and then a further category of even less beer-like drinks (sanrui, I think they are called) to be both cheap and a huge source of profits for the drinks companies.