Military personnel and folks in general are expected to learn from their mistakes. Note that no cigarette burns were actually inflicted.
The Army does not put privates in charge of determining the policies and treatments of prisoners. There were senior noncoms and officers there, that the Army did give that charge to that were effectively AWOL.
These personnel are effectively just out of HS. There's a whole spectrum of capacity and wisdom to be found in such a group. If they are tossed into a hot war w/o officers that actively ensure that they adhere to the intent and objectives of the US, this sort of thing can be expected to happen.