1. Ameren must supply everyone ( even the non payers). Ameren must pad the bill to cover losses.
2. Ameren by law can not make money on supply. Therefore they have no incentive to look for the lowest rates. By law they must pass on what they buy to consumers with out a mark up.
3. Ameren has more overhead than the suppliers. Why does Ameren deliver the competion? Because its the law.