They want to keep production rolling continuously to avoid the lag of shutting down and starting the machinery back up again, but they also know how few people are around who'll willingly work the graveyard shift so they're basically trying to package it into the job no matter what so that they don't have to negotiate for volunteers, because now it's part of the job description contract you signed!
Correct. I walked off a job for missing mandatory overtime (12 hours, 7 days), for having dared had the flu, probably from lack of sleep and sun, with a doctor's note. Went straight to another factory, explained what happened (this was a Wednesday), was told to rest until Monday. Started on Monday five eight hour days, a hiring bonus that covered my lost wages, and immediately made more production. That factory only ran one shift, too. They knew the value of happy, healthy employees. Unfortunately, that happened in the late eighties. So sorry for factory worker abuse now.