As U.S. competition authorities ponder whether age-old antitrust laws should be modernized to apply to tech giants, a first-order question is: What existing antitrust laws apply to their conduct? A formerly formidable tool that has been defanged through lax enforcement is the Robinson–Patman Act (RPA). Passed by Congress in 1936, the RPA was drafted in response to a growing public concern that large chain stores were squeezing out small businesses....