"Why is it than when the US conducts missile launches its OK but when NK conducts launches is illegal?"
United Nations Security Council is the body that decided it was not ok and second geographically speaking it does not have many paths to fly that do not potentially fly over another country. Literally any long range testing would most likely fly over Japan. Unless you think North Korea would fly it over Russian or Chinese territory.
Objectively speaking, the United States has a vast body of water on both coasts. Russia has the artic, India has the Indian ocean and China has the east China sea sort of (a lot of shipping lanes there) or there own land as they are large enough for that.
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