How is this different than our hiring known Nazis to translate German documents during WWII?
Because people working with former Nazis knew that they were Nazis.
Because people working with Germans who said that they had never been Nazis or Nazi sympathizers, knew to take their protestations with a box of salt.
Because the Nazi Reich had been defeated and lay prostrate under our shiny jump-boots.
Sample conversation:
"No, Hans, you may not use the special library containing classified information about numerous sources, means, and methods."