From the EFF:

In what can only be described as a results-oriented opinion, the court found Skinner had no reasonable expectation of privacy in the cell phone location data because “if a tool used to transport contraband gives off a signal that can be tracked for location, certainly the police can track the signal.” Otherwise, “technology would help criminals but not the police.” In other words, because cell phones can be used to commit crimes, there can’t be any Fourth Amendment privacy rights in them. If this sounds like an over-simplistic description of the legal reasoning in an opinion we disagree with, the sad reality is that the court’s conclusion really did boil down to this shallow understanding of the law.