The authors have already taken a bit of a beating about this on Twitter from the agriculturally-literate-intelligentsia. Why? Because these sentences give the incorrect impression that farm policy is a major contributor to obesity. That's not saying farm policies aren't inefficient, only that they do not have the effects many people claim they do.
Why would farmers support policies that would make commodities "exceptionally cheap" and thus lower their profits? Yes, there are some policies that likely increase production beyond what would happen in an un-distorted market, but there are other policies that reduce production. Take corn, for example, which is the largest agricultural crop in the U.S. in terms of value of production. The existence of subsided crop insurance subsidies and commodity programs might increase the tendency to produce more than would otherwise be the case, but ethanol policies from the EPA re-direct much of that production to fuel rather than food. Moreover, there are countervailing policies such as the Conservation Reserve Program (CRP), which remove land from production. In addition, sugar policies push the price of sugar up, not down.
The authors also point to processed food as another big evil, but in so doing they (correctly) undercut the argument that farm policy is a major culprit. How so? Well, for every $1 we spend on food, only about $0.15 results because of the cost of the farm product. The other 85% of the cost is from transportation, processing, packaging, marketing, retailing, etc. As a result, changes in farm commodity prices have relatively small impacts on retail prices.
Fruits and vegetables are indeed more expensive than many commodity crops, but that's because of biology not policy (see more on that here and here). Here's what I wrote in one of those posts: