Turnouts are always optional as long as you are not below a posted minimum speed. Where there are many miles of curves and hills, its a nice gesture to pull over and let behind traffic  pass but nothing more than that. It's not a legal or moral obligation and Frankly I don't feel too compelled to turn out and almost never if I am going the speed limit. It's almost always a scenic area and locals and the impatient need to understand that some may have driven hundreds of miles to take that route, relax and enjoy the scenery.