If Ou had already been inside the U.S. border, law enforcement officers would have needed a warrant to search his smartphones to comply with a 2014 Supreme Court ruling. But the journalist learned the hard way that the same rules don't apply at the border, where the government claims the right to search electronic devices without a warrant or any suspicion of wrongdoing.
So it seems neither American laws nor the laws of the country, where they are based, apply to these preclearance officers. They can simply do what they want. Only more reason to object the expansion of these preclearance facilities to additional countries.