They were called that becase an (electric) camera would scan paper and transfer the eletrical signals to electrically charge the toner so the toner sticks to paper in the exact same points and a heater melts the toner to adhere permanently.

The inventor, Chester Carlson, originally called it "ElectroPhotography" but later the Haloid corporation took interest and licensed the patent from Carlson and recoined the term to "Xerography" and the machines they produced were called 'Xerox' machines. Later the company was renamed to Xerox Corp.