Checking out this item, I asked myself: Why do they only work in poorly lit areas?

If I undestood that right, they´re thermal sights. Should their efficiency not be dependent on the temperature in the surrounding area?

Well lit areas tend to be warmer, making the sight too muddied by waste heat sources to help much. Obviously, this shouldn't be true everywhere, so in cold climes it would probably still be useful.

It's still a weird rule.

If they are only passive IR they should most definately work regardless of lighting. Maybe they also incorporate active IR and light amplification to provide a more detailed view than just a roughly man-sized hot blob in a black void. These sensors could be overwhelmed by bright light, triggering safety cut-outs that effectively switch off the whole unit. After all, Preysense is about the best night-vision gear you can get, so it should incorporate all sorts of sensor systems.

Or maybe the machine spirit is lazy and turns off in lit areas?