They don't admit new bodies, but they do admit new ideas from everywhere. With modern methods of communication, ideas no longer have to be brought into a country wrapped in human bodies. A wise nation admits just the ideas, leaving the bodies to be taken care of by the nations that produced them. This is the way of survival. Before panicking , let us look at the word global.
Some problems are certainly global.
Take acid rain. Take the green house effect. Both cases involve the atmosphere, which is forever distributed and redistributed over the entire globe. Admittedly, it will be difficult to produce the global cooperation that is needed to solve such global problems, but no lesser solutions will work. Now, let's look at the potholes in the streets.