Cloud argues, "the emblematic moment of the period from 1955 through the 1980s in American labor was the tragic PATCO strike in 1981." Most unions were strongly opposed to Reagan in the 1980 presidential election, despite the fact that Reagan remains the only union leader  to become President. On August 3, 1981, the Professional Air Traffic Controllers Organization  union—which had supported Reagan—rejected the government's pay raise offer and sent its 16,000 members out on strike to shut down the nation's commercial airlines. They demanded a reduction in the workweek to 32 from 40 hours, a $10,000 bonus, pay raises up to 40%, and early retirement. Federal law forbade such a strike, and the Transportation department implemented a backup plan  to keep the system running. The strikers were given 48 hours to return to work, else they would be fired and banned from ever again working in a federal capacity. A fourth of the strikers came back to work, but 13,000 did not. The strike collapsed, PATCO vanished, and the union movement as a whole suffered a major reversal, which accelerated the decline of membership across the board in the private sector. Schulman and Zelizer argue that the breaking of PATCO, "sent shock waves through the entire U.S. labor relations regime.... strike rates plummeted, and union power sharply declined." Unions suffered a continual decline of power during the Reagan administration, with a concomitant effect on wages. The average first-year raise  fell from 9.8% to 1.2%; in manufacturing, raises fell from 7.2% to negative 1.2%. Salaries of unionized workers also fell relative to non-union workers. Women and blacks suffered more from these trends.
Question: What enabled the Transportation department to fire the strikers if they did not return to work within 48 hours?
Answer: law forbade such a strike