Neil the Ethical Werewolf points out
the huge skew in how US farm subsidies are distributed -- over 70% go to meat and dairy production, and much of the rest goes to empty starches and sugars. This makes those foods disproportionately cheaper, and thus contributes to the skew in American diets toward eating too much meat and low-quality carbs. Neil sees the problem here as one of nutrition, particularly for the poor who can't afford to fight the subsidy current. I don't want to dismiss that concern, but I do want to point out another effect: the harm to animal welfare and the environment.
In discussing the animal welfare/rights or the environmental argument against consuming meat (or at least factory-farmed meat), the options for action typically wind up focusing on personal choice, i.e., becoming a veg(etari)an. The obvious public policy instrument would be some kind of ban on the production and/or consumption of (some kinds of) meat -- though you'll almost never hear anyone advocating that. But here we have a case in which some progress could be made simply by rearranging our subsidy system.