The soldiers in Vietnam were specifically trained to do that and the M16 rifles of that era were specifically designed. There is a long history behind it. They did an exhaustive study of 3 million battlefield reports from World War II and they concluded aimed accurate fire was useless as targets were on the move. They wanted to equip every infantry troop with an automatic rifle. The World War II German army had considerable success by equipping whole platoons with sub-machine guns and that was studied too.
Here is a link to the history that describes Project SALVO. The "spray and pray" concept dates back to World War II with German infantry units being equipped with the MP40.
http://www.bravocompanyusa.com/Articles.asp?ID=125