By all accounts, the Federal Aviation Administration’s "tarmac rule" has dramatically reduced the number of passengers who are stuck inside an aircraft on the ground for three hours or more. Violations of the rule, which went into effect last April, can cost airlines $27,500 per passenger, or $2.75 million for a planeload of 100 people going nowhere fast. In fact, there were just three cases nationwide of three-hour tarmac delays in December — compared with 34 the previous December, according to the federal Department of Transportation, the FAA’s parent agency. But critics say an unintended consequence of the rule is...