>Because it lets white Americans...[shift] the focus away from them and their ancestors.
Not every white American had ancestors that owned slaves. In fact, most white Americans had nothing to do with slavery--much like today, only the top 1.5% (1) or so of Americans enjoyed any of the benefits stereotypically ascribed to "white people." Poor white people were left to scrounge for low income jobs and educational funds, much as they are today.
Blaming all whites for slavery is much like blaming all Muslims for 9/11. From the past to the present, only the rich have benefited from slaves--as can be seen by the fact that the first slave owner in America was actually a rich black man. (2)
1. [http://en.wikipedia.org/wiki/History_of_slavery_in_the_Unite...]
2. [http://en.wikipedia.org/wiki/Anthony_Johnson_%28colonist%29]