It seems they shut the inverters down because a legal dispute. So the reason is the law.
The power inverters were *not their property*. Remotely accessing them, without authorization and with the intent of disabling them, is a textbook CFAA felony.
Fuck 'em. Isolate your local net from the world and only let through devices you trust. Plenty of ways to do that, even at low expense. But you will have to make the effort or pay someone else to do it.
You buy a device from an intermediary and it phones home to a foreign jurisdiction. That sucks but I'm not sure what recourse you can realistically expect.