I think they're looking over the social aspect of this. Areas with highest birthrates tend to be the most uneducated ones, and thus you will have a greater amount of people who will more likely make irresponsible decisions for the planet's sustainability. Look at how many people still don't believe in global warming, and how fiercely they're opposed to the government taking initiatives to solve these problems. Look at a list of countries by population growth rate, and look at a list of countries by how much they recycle -- you'll see a compelling correlation.