Sorry for asking what I could Google for myself - do they not charge ISP customers for data received? Are they double, or even triple-dipping?

Of course they're double dipping.

Korean ISPs can't charge the customers more, because many of the customers (both landline and mobile) have been promised "unlimited" data. There'd be a colossal shitstorm if they ever tried to take that back. So they look for easier prey: foreign video streaming platforms.

Yet in the US they took it away and we thanked them for the honor. It seems to be coming back but not quite these days.