Because racial discrimination is wrong, always, everywhere, forever. A White Europe policy, to adopt an antipodean phrase, is simply not an option. So, other than racism, what's the alternative to multiculturalism?
In a word:
Assimilationism. Government policies designed to promote domestic values and assist immigrants in acclimating to their new home. Not letting them simply form their own enclaves and ignore the broader society. That's not to suggest that they don't get to keep their own religion and elements of their culture, so long as they don't conflict with the laws and broader mores of our society. The "melting pot" approach, so to speak.
That's the way we have been doing it in the USA for many years. And for the most part, it has worked. Just FYI, I live in Northern New Jersey which has a significant immigrant population from many different regions/countries. I can tell you firsthand that this approach works far better than multiculturalism.
I don't see anyone complaining about the relatively closed immigration policies of China, South Korea, Japan, United Arab Emirates, etc., and those are all (relatively) wealthy countries.
Absolutely I disagree with their policy, and they must change it immediately.
Again, you are assuming that a policy which would work in a wealthy, developed, ethnically-mixed country (like the USA) is appropriate for all. That's a rather blinkered assumption.
What about smaller countries who are desperately trying to preserve their culture (like Bhutan, let's say)?
Oh, and you never did answer my question about the Native Americans.
I'm all for freedom of religion, dress, food, etc. But I think immigrants should at least examine the values of their host countries and try to adopt them. As Shakespeare once wrote, "When in Rome, do as the Romans do."
Red herring; we're talking about what the government should do, not immigrants.
The two things are connected though. You are Australian, right? (I gathered as much from the allusion to the "White Australia" policy, which is not what I am advocating here anyway.) I have always wanted to visit Australia. How would you like it if a bunch of obnoxious Teabagger white Americans moved into your country and started complaining about everything and making all kinds of political demands, like the elimination of your public healthcare system because it is "TEH SOSHULIZUM!!1!" ? Shit, it's bad enough that you have to put up with the whinging Poms, right?