In this case the law is just fine. The State Supreme Court ruled, essentially, that you can't put an object in someone's car and accuse them of theft if they remove it. The court invalidated search warrants issued based on that faulty assertion.

The argument was more that it was a constitutional violation - search of property without probable cause. The police conveniently assumed the device was stolen, and used that as a pretext to search someone's home and advance an investigation that was the reason for the tracker being fitted in the first place.

They didn't consider that the device might have been lost, and the suggestion that it was stolen was never a good one. ("One used tracking device for sale. Contact owner...")

It'd be interesting to see how this might shake out related to lost-and-found precedent - I bet there's some insight there. If you find a sweater abandoned in the woods and take it home I'd assume that isn't stealing unless there was a reasonable case to be made that the article of clothing was recently discarded - but if someone comes looking for it or puts up fliers that may change the math on that.

This is essentially the same idea - something was left on your car, potentially maliciously even.

It's an interesting point, though I wonder how the intentionality would play into it. Malicious or not, it's hard to argue that someone who places a tracker (or a bomb!) on your car has "lost" it, as opposed to knowing exactly where it is and wanting it there for specific purposes.