If I were to play God, and had a world full of resources and fertile lands, largely unpopulated, why in the hell would I start putting a bunch of them into an island that's populated by 100+ million people as it is? For no other reason than to fuck with them, I suppose.
It doesn't make any rational sense to do that. Japan's immigration policies make perfect sense from their perspective. They're a small island, with relatively few resources, stacked with people who already struggle enough to co-exist with one another, as it is. They operate under very strict social rules, and any departure from those may potentially cause a structural problem. Hell, we're already seeing that structural problem right now as young men and women have grown more "liberal", and less reproductive to the society.
America's relatively more lax immigration policies worked for them in the past, because they had a big, fertile piece of land that was largely unpopulated. One could ask the question whether those sorts of immigration policies are sustainable though as America's population continues to grow (including the population under poverty line). At some point, you need to start focusing on what you've got.
The problem with many Americans (not saying all) is that they feel the need to impose their standards upon all the others, without taking the circumstances into considerations (see their Middle Eastern campaigns). Nobody has really challenged America's position or whatever it is that they want to do amongst each other, in their own lands (maybe Bin Laden did, but he was just one nutty mountain bandit). Unfortunately, the tendency to make decisions for other people is there, and it has caused a lot of bad shit to go down. Some good, but a lot of bad as well.