Are you looking for a linguistic answer, or a political one? I'll take on the former here! The phrase "the EPA's Midwest chief[,] and an Obama appointee" is appositive to another noun, i.e. "Susan Hedman", and describes it further. Because the phrase is set off by commas, it's not essential to the sentence, but it does add information, and one element of that is to say Susan Hedman was an Obama appointee. (You may be confused by the second comma in the sentence. It's not usual to use a comma before "and" with a pair of noun phrases like that, particularly when they are short. It also miscues the end bracketing of the appositive phrase, and so I would have left it out myself. But it's a question of style, and a writer may use a comma if they feel like it.)