While that doesn't mean that heat is transferred directly to the cabin, it will increase the heat load on the refrigerant loop, decreasing the amount it can cool the cabin. Perhaps they don't explicitly make use of this to avoid using the cabin heater, but any time the powertrain or battery is seeing a lot of heat load, it will impact how much cooling the refrigerant loop can provide to the cabin. So if they're not using it intentionally in this way, it should only take a software update to do so.
Maybe you are forgetting that this isn't an ICE with a non-controlled AC compressor. Tesla can control the compressor to generate the level of cooling that is demanded, up to the maximum of the system, so they don't need to use electricity to create heat just to reduce the amount of cooling the AC system generates.
There is no path for the heat from the battery and motor to make its way in to the cabin. Lets say it is 0* out the AC isn't running since there is no need for cooling, unless the battery/motor need cooling in which case the cabin still isn't using the AC. There just isn't a pathway to get heat from the glycol loop into the cabin. (Well in the Model 3 the glycol does have a loop sort of into the cabin but that is to cool the MCU/APE.)