There's no torque on you ankles when you're snowboarding because your feet are stationary. The only way they bend when boarding is forwards and backwards, a natural motion. With skiing, your feet are independent, and having a 6 foot piece of wood attached to your foot puts tons of torque on your ankles because they are a realtively week joint. You are also bending your ankles side to side when you go on edge. So the hard plastic takes the pressure fron your weak, non-sideways bending ankles by imobilizing them and transfers it to your stonger, more mobile knees and hips.