My guess is that it is typeset for a particular journal of law - whatever is used to officially document SCOTUS decisions, with an appropriate column size for that. The additional electronic publishing is just using the same format but slapping it in the middle of A4/Letter.
It's possible that the margins are meant to be cropped by the printer. Though more likely it's just an optimization of line-length, font size, and ink usage. (since ink costs more than paper)
