It is true that from necessity of nutrition does not follow that starving the farmers is wrong. For example, in order to repel an invasion the troops forage in the farmland and that might cause starvation of the farmers. Or, a natural famine might not be alleviated at any reasonable cost and the farmers starve through fault of no one.
However, this is not the only “is” producing the “ought”. Generally if there are two objective facts: (1) a good is destroyed and (2) an outside actor free to act differently destroys it, then we can say objectively that the actor ought not do that: he is violating a natural law. For example, if the Russian countryside were simply left alone in 1930’s, the Russian farmers would not starve and would have excess produce to feed the rest of the country. Therefore forced collectivization was a moral wrong, — not simply because the farmers starved but because if left alone no one would have starved.
In short, if a good is enjoyed solely through ownership of a resource, then such enjoyment is a right and freely chosen violation of that right is a violation of natural law.
I suppose Double Effect is both one of the most important concepts in natural Law, and one of the most frequently misapplied/misundestood. It's hard to explain the difference between Proportionality and Consequentialism. But it CAN be explained, and when people get it, it's like he cartoon light bulb going in over their heads.
It's indispensable. Every year, I try to teach Double Effect to my n00bie RCIA Catholics.