[Cracker Jack]

Why isn't the victim a "Caucasian-American"?

Or why aren't the perps "blacks"?

This is easier to figure out if "African-American" is taken to be a culture, not a race. Not all blacks are part of the African-American culture. That is how people like Clarence Thomas or JC Watts can be excluded by African-American groups. This is why Bill Clinton could be claimed to be our first "African-American" president. You will find blacks in Africa, the Caribbean, and the Americas. Only in the US do many insist on being called "African-Americans". The group of thugs that beat this kid are definitely of the African-American culture.

[Izzy Dunne]

This is easier to figure out if "African-American" is taken to be a culture, not a race.

OK, so why is their "culture" relevant to anything?

They didn't refer to the victim's "culture" - there's no such thing as a "white culture".