Three hypotheses about race in America:
It's about culture: the divisions are cultural, not racial, except that racial appearance is recognised as a definitive cultural marker.
There's no specifically white culture, that is, culture that white people generally share in and others generally don't. However, there is Irish-American culture, Italian-American culture and so forth.
There's little interest in challenging the "one drop rule" that determines that anyone with any perceptible black African ancestry is black, even though it has its origins in slavery and racist notions of purity and pollution.
Curiously, in Brazil the "one drop rule" goes the other way: one is considered white in Brazil if one has any perceptible white ancestry. People travelling between the two countries sometimes find their race reassigned.
There are people such as Barack Obama and Halle Berry who are considered black in America, but were raised by white parents only and come across to white people at some level not so much as "black" but as "happens to be black". I suspect this made Obama electable in a way that Jesse Jackson wasn't. Berry says she lets people categorise her as they wish; a few years ago she appeared in a PSA ad for the MLK Foundation's "Build the Dream", which shows her being seated in a section of a restaurant in a hypothetical world where segregation never ended (apparently not available on YouTube). I believe it gets its punch from white people subconsciously reading Berry as "one of us".
To recap other criticisms of the Stuff White People Like blog, it's really about America's young upper-middle class, which Lander is both poking fun at and celebrating, plus a little bit of poking fun at white ignorance of other cultures for credibility for its edgy premise. Thus it would exclude non-white people from the prosperous and well-educated set.