ATCs weren't exactly forced to work: they aren't slaves and are free to quit any time. But if they didn't show up for assigned shifts even though they weren't getting paid then they were subject to disciplinary action including termination. Some of them called in sick, or took on temporary second jobs to bring in some cash (obviously a bad thing from a fatigue management standpoint). After the government shutdown they were paid in arrears for all of the hours they worked. It's crazy that Congress plays political games with essential services like ATC.