The Eighth Amendment forbids “excessive fines.” The Supreme Court has defined that term exactly once, in 1998. A fine is unconstitutional when it is “grossly disproportional to the gravity of a defendant’s offense,” the court held in United States v. Bajakajian. In 2019, the court confirmed that the Eighth Amendment restricts not just the federal government from imposing excessive fines, but also states, cities and counties. That is nearly the whole of the doctrine, and the lower courts have split over what “the gravity of a defendant’s offense” means. The justices now can create the yardstick every court must use before calling a punishment excessive.

Ask what Mr. Jouppi did, and the confiscation of the plane looks wildly out of scale. Ask how serious alcohol smuggling is in the abstract, and he loses before the facts are heard. Alaska’s struggle with rural alcohol abuse is real and grievous. Mr. Jouppi was a professional pilot who, a trooper testified, would “have to be blind not to know” what he was loading. Weighing the harms alcohol visits on rural villages — crime, alcoholism, fetal alcohol spectrum disorders, death — the court concluded that “the illegal importation of even a six-pack of beer causes grave societal harm.”

Mr. Jouppi has a legitimate defense. He was a first-time offender, convicted of a misdemeanor over a customer’s beer and given the lightest sentence the law allows. To take a $95,000 airplane on those facts is the very picture of a punishment that does not fit the crime.

The Supreme Court has spent three decades reining in excessive punishments, just not under the Eighth Amendment. The court’s decisions have come under the 14th Amendment’s due process clause, in cases about punitive damages, and the beneficiaries have overwhelmingly been corporations. In one case, the court threw out a multimillion-dollar award against BMW, which had sold a repainted car as new, by evaluating the conduct’s reprehensibility, comparable penalties and the ratio between punishment and harm. Another ruling for State Farm suggested that punitive awards more than nine or 10 times a victim’s actual losses will rarely survive. And a decision in favor of the cigarette company Philip Morris added a rule that reads as if written for Mr. Jouppi: A state may not punish a defendant for harm to people who are not before the court.

Unfortunately for Mr. Jouppi, those rulings do not apply directly to his Eighth Amendment claim. But their insight should. Corporations — BMW, State Farm and Philip Morris — got individualized review and protection from being charged with society’s wider wounds. An 82-year-old pilot, invoking the one clause the Constitution devotes to excessive economic punishment, had the harms of statewide alcoholism laid at his feet.