They do,
it's just not enforced as a "blood" law. It's self imposed now ---->
This is why the One Drop Rule was created in the first place, to keep people like Robin Thicke's son and the guy above from spreading their genes into the "white" community
"Black" in the USA means
"
Black or African American.
A person having origins in any of the Black racial groups of Africa. It includes people who indicate their race as 'Black, African Am.' or provide written entries such as Kenyan, Nigerian, or Haitian."
[16]
and to add to that
"
And so, why do few if any White Americans display a strongly African appearance (have a high melanin index) despite having detectable African admixture? Because those Americans who “look Black” are assigned involuntarily to the Black endogamous group, whatever their genetic admixture. The scatter diagrams of the two endogamous U.S. groups are not symmetrical because the selection process acts only upon the White group.
As revealed in court records, discussed elsewhere, a person of mixed ancestry who “looks European” (like Dr. Shriver or his maternal grandfather) in practice has the option of either adopting a White self-identity, thus joining the White endogamous group or a Black self-identity, thus joining the other group. But a person of mixed ancestry who “looks African” lacks such a choice. U.S. society assigns such a person to membership in the Black endogamous group, like it or not.25
In conclusion, U.S. society has unwittingly applied selection pressure to the color line. The only American families accepted into the White endogamous group have been those whose African admixture just happened not to include the half-dozen alleles for dark skin (or the other physical traits associated with “race”). Since those particular alleles were sifted out of the portion of the White population that originated in biracial families, the relative percentage of the remaining, invisible, African alleles in this population cannot affect skin color. That skin-color does not vary with African genetic admixture among American Whites, despite their measureably recent African admixture, demonstrates and confirms that physical appearance has been an important endogamous group membership criterion throughout U.S. history. It has resulted in genetic selection of the White U.S. population for a European “racial” appearance, regardless of their underlying continent-of-ancestry admixture ratio."
.
.
.
.
.
and followed by "phenotypical one droppers"
‘One-drop rule’ persists
http://news.harvard.edu/gazette/story/2010/12/‘one-drop-rule’-persists/