Its harmful effects were not known at the time

The US military were first informed one of Agent Orange's constituents was contaminated with a toxic substance in the 1950! They may not have known the full effects when they started using it but they did know it was not "pure herbicide".

Agent Orange was used between 1961 and 1971. It stretches the imagination somewhat to believe that the US were not aware of its effects on the civilian population well before 1971.