I am going to say it's based off of the fact that if they couldn't fit a DL drive in the 15" MBP, how could they fit one in the MacBook. Of course, I could always be wrong, and in the future, you probably could. But to my common sense, if they couldn't fit one in the 15" MBP, why do you think they could fit it in the 13" MB, which has very similar dimensions (most notably thickness and the biggest problem: width).