The matter was decided on following two appeals made by two individuals who were ultimately convicted of crimes based on the information the police discovered in/thanks to their smart phones at the time of their arrest.
In the judges' opinion, perusing the digital contents of a suspect's phone without a warrant equals infringing on his or her fourth amendment protections against unlawful search and seizure.
"Cell phones differ in both a quantitative and a qualitative sense from other objects that might be carried on an arrestee’s per-son. Notably, modern cell phones have an immense storage capacity," they noted. "Before cell phones, a search of a person was limited by physical realities and generally constituted only a narrow intrusion on privacy. But cell phones can store millions of pages of text, thousands of pictures, or hundreds of videos."
"This has several interrelated privacy consequences," they pointed out. "First, a cell phone collects in one place many distinct types of information that reveal much more in combination than any isolated record. Second, the phone’s capacity allows even just one type of information to convey far more than previously possible. Third, data on the phone can date back for years. In addition, an element of pervasiveness characterizes cell phones but not physical records. A decade ago officers might have occasionally stumbled across a highly personal item such as a diary, but today many of the more than 90% of American adults who own cell phones keep on their person a digital record of nearly every aspect of their lives."
"Digital data stored on a cell phone cannot itself be used as a weapon to harm an arresting officer or to effectuate the arrestee’s escape," they also decided, and additionally ruled that law enforcement's fear that incriminating data could be remotely wiped out before a warrant is secured is unfounded, as they can simply remove the phone's battery or put it in small Faraday cages until they get the warrant.
"We cannot deny that our decision today will have an impact on the ability of law enforcement to combat crime. Cell phones have become important tools in facilitating coordination and communication among members of criminal enterprises, and can provide valuable incriminating information about dangerous criminals. Privacy comes at a cost," they noted.
"The fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought. Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is accordingly simple— get a warrant," they concluded.