Edging bunkers was probably originally done so that their 'shape' would not be lost. Sand will sometimes migrate from the trap into the grass and grass will grow into the bunker causing the edge to 'move'. Plus don't forget that the USGA recommended a sizable lip to keep golfers from putting out of bunkers. I remember hearing from several contributors on this site about how the bunker edges at Maidstone had been allowed to move. Basically it sounded like there grass in the bottom of the bunker where originally there was sand. I do not like sharply edged bunkers, we edge ours minimally once in the spring just to keep their basic shape. As far as the rules problem goes it's bullship as far as I'm concerned. Next we'll be edging our maintenance roads and cart paths for the same reason.