You are right... it does appear that the there are a few discretionary spots on the Harvard Law Review for their affirmative actions program, among other things.  However, I read that Obama graduated Magna Cum Laude which would indicate that his grades probably good enough to qualify for the law review.  I'm not sure of the quality of the source on that... but, I would be inclined to believe it. 

It also appears that the president of the law review is selected by his peers.  I doubt they would be motivated by affirmative action to honor someone with that position. 

I am surprised that they could get away with an affirmative action program for law review selection. As I said, the competition is fierce - that must really burn students that barely missed the cut. 

Best,
Robert A. Franco