It is the business' business if you work hours 33-40 anyway. The whole point of the 32 hour workweek is the idea that those extra 8 hours decreases your productivity in the first 32.
Yes, it's still possible you'll work on a side gig / for someone else in the weekend, but that's much less likely than if you have business hours available for that.
I’d argue that outside of the time I’m being paid for, the business has zero influence over what I do with my life.
If I can spend my whole Monday night playing WoW, coming in completely destroyed the next morning, why couldn’t I do some personal work during the day on a Thursday and come in refreshed and happy on Friday.
To be honest, it seems a bit disingenuous to me. A day off should be a day off. What I do with that time is nobodies business.