There's issues like bathroom breaks to deal with. There are also schedule considerations for interfacing with both MAX and SMART services. And beyond that just because the operator isn't running the unit doesn't mean he's not working. He or she isn't at home, or playing a computer game, or what have you, just as a conductor sitting on a locomotive staring at a red signal for 6 hours is still working. Work and exertion are not the same thing.