You do not specify which trim level Impala you own. The factory installed Remote Start System operates differently if you have an LS or an LT/LTZ/SS model.
The base LS models do not have the external temperature display sensor. Remote Start on an LS requires the owner to set the climate control system to the settings they want before turning the car off. In the winter, this would mean turning on the heat before you park the car. In the summer, you would need to turn on the A/C before shutting the car off. Then, when you remote start the next time, the car will return to those settings. However, using Remote Start on an LS will never turn the rear window defroster on.
LT/LTZ/SS models have the external temperature display sensor. The Remote Start system on these models checks the outside temperature and then sets the climate control system itself. In the winter, the remote start will turn the heat on if it detects the external temperature is at or below a certain level. It will also engage the rear window defroster. In the summer, the car will activate the air conditioner automatically if it detects the external temperature is at or above a certain level. The rear window defroster will not come on automatically when it is warm outside.
Keep in mind that in all cases it will take at least 5 minutes for the car to truly cool down/warm up inside after remote starting. If you remote start and then jump right in, it will not feel like the car is using any specific cooling or heating settings.