I suspect that they don't consider the area behind your car a Blind Spot since you can easily see it via the mirrors.
I think the purpose of the Blind Spot Information System is to warn you when someone is actually in your blind spot (more beside your car where you would have to turn your head to see) that you cannot easily see with your mirrors?
Well, part of the blind spot is actually fully behind, an area not covered by the inside mirror, and not covered by the outside mirror unless it's aimed way to the left.
Since the BLIS sensors are part of the Cross-traffic detection system, I'll ask if, when in reverse, your car is seeing approaching traffic from the driver's side. For example, if you are in a parking lot and a car drives down the aisle behind you. If that isn't working as it should there may be an issue with the sensor.