I think it's the way its written. It's confusing.
Congress subsidized farmers to grow corn by paying oil companies a 52 cents per gallon subsidy
IMHO, I read it as Congress paying oil companies 52 cents a gallon to blend the ethanol and gasoline, thereby increasing the price of corn due to demand. Kind of a back-door 'subsidy' for farmers by increasing the market.
I really don't see it saying farmers were paid that amount.
You got it right — It’s a back door subsidy paid to oil companies and passed on to farmers when they purchase corn.