I recently got a Planet Bike Superflash taillight (works great). The mounting hardware is set up so that if you put it on a chainstay, the light will be on the right side of the stay. That's how you'd want it if you were mounting it on the right side (drive side) of your bike. In fact, an illustration on the package shows it mounted on the right side.
Seems obvious to me that, at least in this country, one would want to have it mounted on the left (the traffic side).
No big deal, but why do you think they set it up that way.