What makes South Dakota different
The interstates here are posted at the top of the national range, and that interacts with the schedule in a way drivers underestimate. Speeds that feel ordinary on a road posted that high are, in margin terms, well into the brackets that carry a real point value.
The offsetting factor is enforcement density. South Dakota has few people spread across a large area, and patrol coverage is correspondingly thin outside the main corridors — which makes a citation less likely and does nothing at all to make one cheaper when it arrives.