Severe myopia can be a risk for retinal detachment, in younger people, i.e. early 20's and under. Even if this was a consideration I can't see why it would be relevant, detachment does not always lead to blindness, if caught in time it can be treated. Also, you would need to make playing contact sports a precluding factor!
Also, if conditions that could lead to costly absences were a deciding factor and not just safety, the medicals would need to take into account risk factors like race, gender and lifestyle, which rightly they don't!