Right after my husband retired from the Navy, he got a job driving cabs. He worked 12 hours a day, 7 days a week and he was impossible to live with--grumpy and irritable all the time. God gave us a day off because we obviously need at least one day a week off to relax. I know that from experience.
Cab companies can split their hours of work between more people without any change in the law. Having a government-mandated day of rest would effectively say that
nobody can drive a cab on Sunday (or Tuesday, or whatever day you pick), even a part-time driver who only works weekends.
Actually, I guess this doesn't apply to cabbies, but truckers have rules and laws about hours of work (both in a day and in a week, as well as hours of rest between shifts); the implication of them is that a trucker would never be able to legally work a schedule like your husband's - they have government-mandated rest time without the government specifying
which day they take off (though, to be fair, this rest time might be half a continent away from their home and family). There are solutions that provide the benefits of rest that don't involve saying "nobody works on Sunday!"