History of "Asian American" / "African American" nomenclature

Why are some Americans named to indicate their ancestry? It is not common to say, German Americans, or Russian Americans; however, African American, and Asian American are accepted nomenclatures. Even President Obama's Wikipedia entry reads, "He is the first African American to hold the office."

What is the origin / reason for this?

Update: "I am an American. Black. Conservative. I don't use African-American, because I'm American, I'm black and I'm conservative. I don't like people trying to label me. African- American is socially acceptable for some people, but I am not some people." -Herman Cain

"African American" is a Politically Correct way of avoiding to say "Black" which replaced "Colored", which in turn replaced the infamous "N-word", because this was perceived to be too discriminatory. It was coined after the terms "Anglo American" and "Irish American". It is also more precise because you can have a dark skin and not be of African ancestry (see Papua New Guinea or Andaman for instance).

"Asian American" (replacing "Oriental") and "Native American" (replacing "Indian") were phrases coined in a similar way.

Neither the German nor the Russian immigrants have been markedly discriminated against as a community. Nevertheless the phrase "German American" and "Russian American" also exist if somewhat less commonly used. Other significant immigrant communities have their corresponding label as well ("Italian American" for instance).