If they have proof that I am in possession of stolen equipment, they are obligated to inform me of that and I will return it, but surely they would have flagged the serial number as stolen in the 4 months before they asked me to register it, or some time in the almost year since then.
I do not believe they are obligated to inform you of that. It's not like when you are driving down the road and get pulled over by police and then they find out through your driver's license, which is linked to your VIN, that it has been reported stolen.
Merchandise is stolen all the time. Serial numbers are reported to police, it gets written on a police report, if the item is recovered it might be returned or it might be auctioned off. The person reporting the theft likely files an insurance claim, the insurance company pays out the claim because there's a police report, etc. A person with the stolen merchandise has to be caught. The people who caught them have to care enough to do something about it. Then if they do, they file a report with police, insurance, or both. It is highly unlikely that possessing one stolen item that you paid under $1500 for and have a paper trail for purchasing legitimately through a reseller will land the purchaser in jail.
If Deye knows the S/N of all the products that were supposedly stolen from somewhere, and it sees them show up all over the world including the USA, the decision to brick those devices is a very bold move. Why not put a 'call support' message up on the screen and see if you can find out where people bought them from, and then go after that 'fence' who is moving the stolen goods. Sounds like you, webbbn, may know who the fence is.
But again, sometimes if you buy something worth 2,000 for only 500, and you know that something doesn't seem right but it works and so you just use it anyway, this is the price one pays for that decision. It is rare that this happens.