This is a variation on the higher sills I mentioned:
In a two wheel vehicle side sliding is likely after a collision. Thus the driver/rider shoulder protection needs vary from 4 wheel vehicles.
Imagine you are inside the enclosed scooter, you are pushed over and are sliding along the ground. Your shoulder is contacting either the door or the side of the fairing. If only "glass" was between you and the road surface, bad things would be happening.