Ummmm... wouldn't it be more productive to ask them, not us?

While any light is better than none, I find it ridiculous that someone would use a white light on the rear. There are conventions about which colors go on which end of a vehicle, and to do otherwise invites confusion. I wouldn't want to be the cyclist who caused a motorist to swerve into the opposite lane because the driver ( rightly so ) thought that there was a motorcycle approaching the wrong way in their lane.