You are obviously trolling, but I'll answer you anyway. Nationalism (especially in the US where the two overlap much less than in other places) refers to citizenship rather than ethnicity. Nobody is suggesting deporting US-born ethnic Africans.
So why have borders? It's so you can have programs that help the local poor. Any kind of welfare/medicine/UBI type program has to have some kind of filter on it or the poor of six continents would sign up and bankrupt the programs, so that filter is citizenship. The same applies to local wages. Allow local companies to hire foreign workers and wages equalize, which for US workers means they go down. And the culture of the US depends on having a middle class.
> What is the national culture of America?
Are you honestly asserting that it doesn't have one? The culture in Silicon Valley is exactly the same as the one in Beijing? The one in Alabama is identical to Russia?