There is no international law shared that controls where airspace is controlled by a nation, that said, each country has it's own idea where it begins and where it doesn't.
That said, to test these missiles, it has to fly over someone and I think we all done it to each other.
North Korea seems to be choosing the safest path for it to fly, too far to the north and it could hit land, south has too much land and islands where it could hit, while it flying east takes it between the two land masses and into open sea.
What North Korea is doing is Terrorism on a world scale, sanctions are not working, they never worked and adding more sanctions now is too late, military action is long overdue.