That’s not quite how it works. It works using the Tesla API, like everything else that controls the car. For “charge on solar” Tesla would regularly need to poll your PW2, and when there is excess solar, send a message to the car to start (or stop) charging, and keep changing the charging rate to track the excess solar. And it would need to do this at quite a high frequency - at least every 5 minutes, if not more.
That is a lot of traffic over the API - significantly more than any other current interaction with the car which tends to be episodic (start a drive, stop a drive, charging). My speculation is it’s a commercial issue with Tesla’s M2M contract with Telstra. If your car was not on WiFi, this traffic would be carried over the cellular network, and probably in excess of what Tesla’s contract permits.
So they’d could limit it to WiFi connected cars only… but what happens if your WiFi or NBN network goes down mid-charge, and your car continues charging until you hit peak electricity rates? Is Tesla opening the door to ridiculous lawsuits from unreasonable customers? Or Tesla might require that you be on Premium Connectivity to use it. Or they solve this issue commercially with Telstra and absorb the extra cost.
I can’t see how the DNSP or any level of Government could have any say in this matter.