You learn to history.
Riddler may have had a mental handicap in tactics, but he wasn't that stupid.
>.>
It wasn't all like "OK GUYS! I am going to exterminate you all so i might as well use some of you in battle ok?" He manipulated some of them and put them on the very front lines so they would be the ones shot down first, only a few of them were smart enough to realize what was happening.
However he didn't use many because he was most likely not wanting to give them the same rights/equality as the Germans.