A good point. The question I would pose is: If, in sudden deceleration from an RTO or more complex arrival (insert your imagination here ...) what happens if the hearing and/or glasses fly off the head?
The person may not hear instructions or be able to read the sign to confirm action before opening the door or - critically - NOT opening the door if instructed by CC due to a problem outside the door. There is also the instruction: "Look through the small viewing window BEFORE opening the door and (if applicable) throwing it out."
So I might be inclined to interpret the guidelines to say NO hearing aids, glasses or contact lenses.