I fail to see how this is the case, but perhaps I'm missing the point you're trying to make? What you're referring to is about colonialism, whereas here the USA is very much incentivized to work within its own borders, on its own food industry. It just so happens that climate change is a global problem as well, and the USA being a large rich country it has more causal influence on the outcome of climate change.
No it's not. The US must lead in reducing environmental destruction and emissions because it leads in producing environmental destruction and emissions (per capita.)
Why does per capita matters? The resources of the Earth are limited, and don't rise with the population. Focusing on the per capita means that we shouldn't take into account overpopulation because they don't consume much, for now. That doesn't sound right.