What is the conflict between people and nature in "The Cold Equations"?

Early within "The Cold Equations," the story reveals a policy by which any discovered stowaways would be thrown into space. However, the short story proceeds to note that "it was a law not of men's choosing but made imperative by the circumstances of the space frontier." This is the core conceit which shapes the story and for which the story is named. It is not as if the pilot wishes to jettison Marilyn; rather, this is presented as an unhappy necessity of space travel given the razor sharp margins under which it operates.
The plot of this story follows Marilyn, who sneaks onto a cargo ship and is unaware of this policy. However, the mission parameters had been carefully calculated ahead of time, allowing for no margin of error. Fuel had been carefully allocated with the mass of the pilot factored in. Introducing a second passenger threw off those calculations, leaving them without enough fuel to reach their destination safely.
That is the essential conflict within the story. When we think about "man" versus "nature," we usually imagine a conflict built around things like harsh climates, weather or temperature conditions, geography, and so on. In this story, however, the conflict is built around something more abstract in that it is focused around those carefully calibrated calculations. Marilyn does not die as punishment for her actions: she dies because the equations require her to die. It's the only option available if the ship is to safely reach its destination.