Stealing back your car can certainly cause problems if done privately by the owner. But here it's the product itself that already came with a bricking mechanism built in and activated it itself. The buyer trusted the seller not to provide a self-bricking phone, and got ripped off. It's never going to affect an innocent phone. It's also no physical items being taken or damaged. No baby is going to be trapped in it, etc.

Actually, there's a very analogous thing for cars - LoJack. Is that wrong too?

It happens with copy protection on software. I've heard of games that become impossible to win if they detect they're pirated. Others that just fail entirely. Is that not OK either?