In 2018, the U.S. Supreme Court ruled in Carpenter v. United States that police must have a warrant to obtain cell phone location data from service providers like AT&T and Verizon. But Nate Wessler, the attorney who argued the Carpenter case and the deputy director of the American Civil Liberties Union’s Speech, Privacy, and Technology Project, told the Observer that companies have justified selling phone location information through data brokers by arguing that mobile ad IDs are anonymous.

"These companies absolutely trot that out as one of their defenses, and it is pure poppycock. … It’s transparently a ridiculous defense, because the entire thing that they’re selling is the ability to track phones and to be able to figure out where particular phones are going," Wessler said.