Yes, I spoke incorrectly (or, rather, incompletely) when I mused about price supports for meat. What I should have said was that the government has huge price supports for fast food, because we're talking beef, wheat, corn and soy all in one package (w.r.t. the Big Mac), not to mention the welfare and food stamps required to supplement the wages of the workers for them to pay rent and sustain themselves. None of these things is a consideration in the price of a barrel of crude.
I can think of some government policies that might make fast food less attractive than alternatives, like minimum wage and crazy fees, but I bet you're right, most of the distortion favors McDonald's. Why on Earth would government promote less healthy food at the expense of healthier alternatives? Oh right, because politicians respond to incentives, like everyone else.