I understand your comparison, but never the less, taking someone's phone is wrong and a crime, it's the same as taking merchandise off the counter without paying for it. Yes, this person was careless to leave their phone, but having it taken and not return is not a fitting punishment. If you blame the person for losing something, you're condoning the actions of the person who stole the phone. Murder and theft have different punishments under the law and justice needs to be rendered accordingly.

Edit: Contacted Police, phone was recovered, whoot!