The impetus to eliminate open-air drug markets has historically come from law-abiding residents of minority neighborhoods, as books by both Michael Fortner and James Forman have documented. In 1973 a Harlem pastor named Oberia D. Dempsey called for mandatory life sentences for heroin and cocaine dealers, because the “pusher is cruel, inhuman and ungodly. . . . He knows that he’s committing genocide but he doesn’t care.” In 1986 Brooklyn Congressman Major Owens introduced a bill to increase federal crack penalties. “None of the press accounts really have exaggerated what is actually going on,” he said. In 1989 Atlanta Mayor Maynard Jackson pledged to make “the drug dealer’s teeth rattle” and proposed seizing dealers’ assets.

Today people living under the scourge of open-air drug dealing still face the constant threat of violence. Only months ago Chicago mourned 11-year-old Takiya Holmes, struck by a stray bullet fired by a 19-year-old marijuana dealer. Former FBI Director James Comey once described the aftermath of a raid in northwest Arkansas that busted 70 drug traffickers. “As our SWAT teams stood in the street following the arrests of the defendants,” he said in a 2015 speech, “they were met by applause, hugs and offers of food from the good people of that besieged community.” The town was predominantly black, and so were nearly all the drug dealers.

The argument that Mr. Sessions’s order penalizes addiction also falls flat. For a mandatory federal sentence to come into play, a dealer has to be caught with an amount of drugs that clearly reveals large-scale trafficking. To trigger a mandatory 10-year sentence, a heroin trafficker, for example, must be caught with a kilogram of the drug, a quantity that represents 10,000 doses and currently has a street value of at least $100,000.