Typically, people who have diets high in saturated fat have an increased risk of heart disease. Those who replace saturated fat in their diets with unsaturated fat decrease their risk of heart disease. Ttherefore, people who eat a lot of saturated fat can lower their risk of heart disease by increasing their intake of unsaturated fat.
The assumption that people who add unsaturated fat to their diets will eat less food that is high in saturated fat most helps to justify the reasoning above.