What a rubbish take. Double nationality is a problem because you cannot be 100% American? Crazy rant.
Not at all. The mistake you appear to be making is conflating a multi-ethnic society with a multicultural society.
Think about it: What is a culture?
Isn't it a collection of customs, behaviours and values that correspond to a particular group of people? And that being the case what happens when those things are in direct conflict to that of the national culture? It really doesn't make any sense because ultimately something needs to take primacy, especially under the law.
The whole idea of nationhood is built upon uniting all those living within the bounds of a certain polity or territory under a single umbrella of shared values. Its vital to the integrity of the whole that bonds be shared such that if a threat from without or within threatens to seize power, everyone comes together for the common good.
A nation without shared values fractures and dissolves. The mixture of ethnicities within its bounds is irrelevant, because those differences are immutable, not political.
Of course post-modernist philosophy disputes all of this, and critical theory as a derivative is a means of placing this intellectual masturbation into political action.