Cheap sunglasses can cause fatigue, as poorly manufactured optics are only accurate in the center of vision. If things on the edge of the optics are out of focus, this can cause the user to overcompensate. First, the brain recognizes that the glasses work best only in the center. The wearer will turn their head rather than roll their eyeballs. Not only is this less efficient, but it could cause an officer to look away from a greater threat.
Wearing eye protection keeps officers from common hazards like windy conditions to unique hazards like having a branch flipped in the face during a foot pursuit. The highway patrol officer who bends over a hissing radiator can work another day because of lens technology. An officer can return to work after a broken bone or two, but a moderate eye injury can end a career.
Tactical eyewear has always been fashionable. But in a profession like law enforcement, it should be common sense to wear glasses. In fact, it should be mandatory.