Likely because they have monitored ever North Korean missile launch and they know these are ballistic missiles. That means they follow a parabolic trajectory, and that means that before they go very far, a reasonably accurate landing zone can be calculated.

The USA and Japan (and China and Russia) likely knew within seconds that it would land in the sea and they even knew how high it would fly over a low population area in Japan.

It's like a car full of young punks stopped at a red light beside a police cruiser and when the light goes green they burn rubber and peel away from the intersection first, but never exceed the speed limit. Very provocative, but the police have real crime to deal with, so they ignore it.