You might say that multiculturalism is a cultural value in itself, and perhaps some might value multiculturalism more than the survival of their culture.

My view is that syncretic cultures are dominant because they are capable of absorbing other cultures and adapting to change, whereas rigid and intolerant cultures can only last as long as their particular moires are relevant.

And in the same vein, Liberals show no hesitation in lambasting other people with markedly different values (such as central/southern U.S.) who they lump in as "the same culture" and so consider a valid target.

I think Scott Alexander touched on this in his "I tolerate everyone but the outgroup" post. He noted that it is relatively easy to get along with those very different from you, particularly when you're uniting against your proximal enemy. He pointed out that even the racist Nazis allied with the Japanese against other Europeans. This isn't tolerance; it's just another way to gain power in a struggle against old foes.

Liberals in the USA were defined by their willingness to push their values, beliefs onto others. That was the New Deal. The War Machine got their profiteering and Labor got their middle class. Uniting left and right, over the objections of the isolationist (minor caucus within the Republican coalition).

That Cold War ear consensus was mooted by the fall of the USSR. A new consensus has not emerged (been forged). Nowadays, we of the left are more aware of blowback, and so are more hesitate to support Gun Boat Diplomacy.