That is partly because during the Cold War, the USSR made a lot of anti-American propaganda in order to court allies from former colonial nations in Latin America, Africa, and Asia. Often these nations had very large non-white constituencies.
Their knowledge of things on the ground completely lacks subtlety and an understanding of the various factors at play, like migration patterns and class.
As I'm sure you know but they don't, in most cases, as far as where people live, it's a function of geography and "class", not "race". My husband, whose parents were working class, went to a high school and thus lived in a neighborhood where 30% of the kids were black, among which were some of his best friends.
If you're on welfare, you don't live in working class neighborhoods, much less middle class ones. If you're on welfare and live in a large city, a lot of the people around you will be black and Hispanic, although not all, depending on the city. In the American southwest, there will be lots of unskilled Hispanics. In the northern states you'll have a lot of blacks because that's where they migrated when the factories were humming. It seemed like a better bet than the south, where attitudes were also different.
If you're on welfare in rural areas like the "hollers" of the Appalachians, everybody will be white, as they will be in rural Montana. You won't find many black people there at all. It's ranch country. The concentration of black people in the mountain states is very small. It's even small in the Pacific Northwest. That's white and East Asian country.
If you're a black executive or doctor or lawyer and you choose to do so you can live in the mostly white upper class neighborhoods and your children will go to those schools. Yes, they'll be in the minority, but people shouldn't forget that blacks only represent about 12% of the population anyway.
My children went to public schools where, yes, the majority were white, but there were quite a few kids of East Asian or South Asian ancestry and yes, a few black families.
All of this has nothing to do with government racism or even individual racism. It has to do with migration patterns, education levels, and economics. If you didn't finish high school, or did but are only capable of minimum wage jobs, you're not going to be living in middle class or upper middle class neighborhoods. That's just reality. My closest friend is Jamaican. I would have loved to have her living closer by, but she and her husband are social workers. They can't afford it. It has nothing to do with race.
This is why you have to be very careful with statistics. They give you national averages of, say, how many minorities live in "mixed" communities. They don't tell you why.
I find it particularly galling when such criticisms come from countries where the "segregation" is infinitely worse. What about France, or Belgium, or Germany or even England? How "mixed" are their communities? Or do all the "non-native ancestry" people wind up all living together.