If hackers can "affect sensor readings" before they feed into the control system, sure. But that is not read only access to the sensor... The trivially simple example is a webcam streaming the thermostat output. The hacker can watch that stream all day - it's not gonna cause any issues with the refrigerator. Even if the hacker had control of the stream, worst they could do is disrupt any monitoring system that uses the stream (e.g. make it appear that there is a fake issue or mask a real one) - but that is significantly lower risk than having remote access to trigger a semipermanent defrost cycle that spoils all the food.