Stopping in itself is not for data collection, it's for safety/liability: they're not yet feeling good enough about detecting light state to base driving decisions on it. In theory, storing clips containing traffic lights and comparing the detection result to your actions (tell the car to proceed because the light was green or allow it to stop because it was red) should allow them to validate and improve light state detection. I don't know whether they are actually doing this, but I did notice my car uploading several GB to Tesla after every drive with the feature enabled.