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...")
