Only because it was an American engaged in warfare against the United States. They try that with an American overseas who is not linked to an organization at war with the US or an American within the United States, then it becomes a crime.
It was a crime either way. U.S. statutes don’t make politically correct distinctions.
And liberals are paranoid....
Did you see the PSA's put out by Homeland Security? They showed 'terrorists' as being white middle class citizens. People who looked like they could have attended a Tea Party. Do you really want to give Janet, Holder and Obama the right to kill off citizens they think might take power away from them at the ballot box?