The alpha dog of the group, likely the 35-year old Sgt. Gruner, (who, by the way, was already in trouble with the law for beating his wife), decided to have some fun with the camel-jockeys, and the rest of the weekend warriors went along, lest they be labeled "pussies."
That is, except for one, who couldn't stand the stain on his honor.
Those in charge of Gruner either looked the other way (the more cynical view) or they were just overwhelmed with the task of thousands of ex-Baathists in custody and left each unit to run itself.
That's what I think happened.