If it quacks like a duck...

For 2 reasons: 1. Motorists have been trained to know that a white light indicates the front of the vehicle. Just as they have been trained to know that a red light means the back of the vehicle. Putting a white light on facing rear sends the wrong signal and confuses the drivers about the disposition of a bicycle. Confusion is never good.

Putting a red light on the front of a bicycle (yes, I've seen this bit of idiocy) tells them that they are over taking a vehicle from behind. If the cyclist is riding the right way, an on-coming vehicle may assume that the bike is a salmon and turn across their path. Squished cyclist.

2. A rear facing white light is likely to blind a driver. If the light is facing forward, the cyclist's position on the road will almost never result in enough light spill to blind oncoming traffic. The distance from the cyclist's light to the driver is just too wide. With a white light facing rear, the motorist is much closer to the light (~4' vs 15' or more) and the light probably isn't going to be aimed as well to take advantage of the beam.