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."
We didn't hire Nazis to translate German documents during WWII.
Many Germans, Italians and Japanese Americans spent the war in internment camps, not "working for" their sworn enemy, us. (Not saying that everyone who was interned was an enemy, but during a shootin' war it was prudent to avoid the unnecessary risk).
We haven't always been this stupid!