I was thinking the same thing - they are different products so the reviews should be separate. I almost didn't buy it because I accidentally read a review for the extended edition instead and thought the theatrical films were split over two discs.
I've seen similar lumping together of different, but related, products elsewhere in the Amazon empire. 1 review per product set which I suppose slows down attempts to manipulate ratings. Since you can review stuff you didn't buy via an Amazon website the whole system can get a bit dodgy.