Well if I needed any convincing that assimilation of whites into a general culture in the US and abandonment of the labels they were born with is a good thing, this weekend would've done it.
(For why, compare the reaction to this in the US to the attitudes in ethnostates in Europe. Americans are standing against this specifically BECAUSE of the destruction and annihilation of culutre, they no longer have a culture to "defend")
Rather it is in large part because cultural pluralism is central to a national narrative.
There's a conservative argument against cultural pluralism, that a nation rightly belongs to or ought to be unified by a particular ethnoculture. And one belonging to a kind of liberalism that says national identity really consists mainly in recognizing it's the current year and certain cultures aren't getting with the program. In European states there is generally the conservative argument, and especially in Western Europe there is the liberal one also - sometimes paradoxically in combination or with the liberal argument used as a more acceptable way of expressing the more conservative sentiment. And because these countries have a civic national identity which shares the same name as the ethnic identity, this is facilitated. In America you see similar arguments, but they are not quite as tenable to as many given historical experience.