Pretty much every country has limits on immigration. It is hardly unique to the US. There are different cultures and traditions in the world. It takes for people to adjust to a new country and the country to adjust to them.
Actually white Americans don't seem to mind immigration from fellow white-majority countries, especially if it's from the developed ones. But that's an understandable phenomenon.
They don't now only because there isn't a lot of it, so it's not a political issue. How many recent German or French immigrants are there in the US? Not enough for anyone to notice. Back when there was large-scale immigration from Europe though, a lot of people minded, and formed parties specifically to agitate against the Irish/Poles/etc.
You see that now in Europe. UKIP isn't complaining only about muslim immigrants (though they complain about them loudest), but also about continental-European immigrants, especially eastern Europeans. I suspect something similar would happen in the U.S. if there were a large influx of European immigrants.
The internet is a good example of a culture than can exist if you do away with artificial barriers between groups of people. Modern immigration policy is not inherently correct and it wasn't devised during an age of hyper connectivity.
If the Internet had to ensure housing, food, and transportation would be available for every participant, we would be a lot more selective about granting access. Open borders made sense in the age of unsettled land, and they make sense when the marginal cost of delivering everything (i.e., only information) is nearly zero.
Whom should you select to let into your country? The same people you select as employees, only more so.
We use "skilled migration" as a proxy for what we really want, which is people with good genes. If you have skills and good genes then you'll enrich the country not only in this generation but in all future generations to come.
So an ideal immigration policy would involve some combination of testing for intelligence, physical fitness and (dare I say it) good looks.
>If the Internet had to ensure housing, food, and transportation would be available for every participant, we would be a lot more selective about granting access.
Unfortunately, the US doesn't ensure anything at all for its own citizens, other than their God-given right to accumulate capital in unlimited amounts.