After further review, 121.651 (4) is cited below. I would imagine that upon reaching the MDA you can't start down from the MDA to landing maybe due to some type of obstruction, but that really doesn't establish anything otherwise they would just make that the final segment??????? So in other words,,,,,,,,,,,,, I dunno? Or maybe because if you wait until the MAP you would be too steep to make a "normal approach and landing"
121.651 (4) When the aircraft is on a straight-in nonprecision approach procedure which incorporates a visual descent point, the aircraft has reached the visual descent point, except where the aircraft is not equipped for or capable of establishing that point, or a descent to the runway cannot be made using normal procedures or rates of descent if descent is delayed until reaching that point.