That is an excellent point. Generally anytime you or your property are not free to move, you are under arrest (or are being detained). Law enforcement must have probable cause to do that. The exception to the rule I stated is if a police officer has reasonable suspicion to believe that you are involved in criminal activity, he may make a temporary detention, called a Terry stop.
However, the GPS analogy the judge used is a poor one. I can drive my car on private property, particularly my own where I do have a reasonable expectation of privacy. Particularly if my car is in an enclosed garage, the police cannot enter my garage without a search warrant to see if it is in there. When the police seize your car and place the GPS in it, it is more akin to a "pen register". (A pen register collects the number you called and not the actual communication you hold) You have no expectation of privacy in the number called, the phone company keeps that record or the owner of the number you are calling will know you called it. Nonetheless, it still takes a court order to get a pen register placed on your telephone.