What kind of dumb ass soldier can't tell the difference between having and firing live ammunition as opposed to dummy ammunition?
Exactly.
Well, there are many accounts of incredible stupidity and inompetence in WWII. They probably used non-combat troops to play the Germans, guys who had no knowledge of weapons or ammunition.
If you read some of the stories in Paul Fussell's book, or in Eugene Sledge's autobiography, you would find this highly credible.
Sledge tells the story of actual combat soldiers being so poorly trained they didn't know to how to pull the pin on a grenade. The Japanese caught the grenades, pulled the pins, and threw them back.