applied to the back surface of the lens to eliminate the annoying reflections of the lenses from the back surface due to any light source behind the wearer. An AR treatment is rarely applied to the front of the lens because the increased transmission of light through the front surface would defeat the purpose of the tint, which is to limit the amount of light passing through the lens. Also, because sunglass lenses are dark, smudges and smearing would be much more obvious on the front surface of an AR sun lens.
Lenses in sunglasses are dark and light doesn't penetrate the lens surface. This effect causes the lenses to be highly reflective and prevents the AR coating from being as affective. A backside AR coating helps reduce reflections of light that enter from behind you and bounce off the surface into your eyes. A glare reducing backside coated lens is much more comfortable than a sunglass lens left uncoated.
AR on the front would tend to make the wearer's eyes visible to others - which many of the people doesn't like.
AR on the front side would tend to highlight or amplify the appearance of smudges or fingerprints on the front of the lenses, as seen by others