Central America's Migrant Caravan and the Notion of Population Control

What Mr. Trump presumably meant is that "the governments of each of these countries seem to have no control over its population." This is a quite remarkable statement. Why would it be good that a government controls its population and can prevent its members from leaving? And if that's good for other countries, is it good in the United States too? Shouldn't it be the population that controls the government, not the other way around?