The petrol they're selling now might have been cheaper, but what they're buying now to replace it with is at the higher cost. Its standard business practice. At my work when the courier company puts up its fuel surcharge to increase the cost of courier tickets we don't do a stocktake of what tickets we had bought at the old price and charge them out accordingly, it would be impossible to keep track of.
Also, I'm skeptical of just how 'green' ethanol really is. What does the tractor that ploughs the field, sows the seeds and harvests the grain run on? And the big trains that take the grain to the processing plants, then the ethanol to where-ever its going?