USA doesn't give two shits about Japan or South Korea...if they did that missile would have been taken out considering initial reports claimed that it was heading for Japan.
So now they damage control the fuck outta the situation and attempt to rationalize why they didn't use their variant of the Iron Shekel against that missile.
https://arstechnica.com/tech-policy/2017/08/n-korea-launches-missile-over-japan-should-us-have-shot-it-down/