There is no federal mandate for E10.
I refer you to the Energy Independence and Security Act of 2007. This does mandate that a defined percentage of our fuels MUST come from renewable sources. This was intended to promote biodiesel and E85 fuels.
Some states do mandate that all gasoline must contain 10% ethanol, but that is up to each individual state.
The REAL reason that almost all fuels contain 10% ethanol is due to the low demand for E85 and the over production of ethanol. The "left over" ethanol is blended into the mainstream fuel to produce E10.