In "Burn," what point about economic realities on the reservation does David make as he steps in the boot prints along the sidewalk on his way home?

David walks in the boot prints because it is easier than breaking a new path, since there is no reliable snow removal service on the reservation. This highlights how few people on the reservation are economically secure, which David comments on when he notes that the only reason most people would be walking into town is for drugs--marijuana or methadone. The few people on the reservation who can afford to do any real shopping in town would be driving there in cars.