It's not exactly your "airspace" at 550km (the reported apogee) as, among other things, there's no air. That's 150km higher than the ISS orbits. It won't be a

It's not exactly your "airspace" at 550km (the reported apogee) as, among other things, there's no air. That's 150km higher than the ISS orbits. It won't be a perfect ballistic curve due to the boost phase not being instantaneous and drag during the endoatmospheric periods, but based on how far it flew and how far it overshot Japan by, the halfway point between launch and landing is just at or just west of the Japanese coast. So a conservative estimate would be that when it crossed over a Japanese land mass, it was at least 500km up. So saying "you can't reasonably allow it" is a little odd when I don't think they could physically have stopped it at that altitude. By the time it dropped down into the reported range of THAAD, it'd have been well past Japan.