1. You are allowed to work more than eight hours per day. It's just that in many industries, employers must pay you overtime if you do so.
2. Even if employers were prohibited from using "willingness to work more than 8 hours per day" as a condition for employment, long workdays would probably soon become the norm.
3. Thus a more feasible way to limit workdays is to constrain employees rather than employers.

To see why, assume that without any restrictions on workday length, workers supply more than 8 hours. Let's say, without loss of generality, that they supply 10. (In other words, the equilibrium quantity supplied is ten.)

If employers can't demand the equilibrium quantity, but they're still willing to pay to get it, then employees will have the incentive to supply it. In their competition for jobs (finding them and keeping them), employees will be supply labor up until the equilibrium quantity, regardless of whether the bosses demand it.

Working more looks good. Everyone knows that; you don't need your boss to tell you. So if there's competition for your spot or for a spot that you want, it would serve you well to work more.

So if your goal is to prevent ten-hour days, you'd better stop people from supplying them.

At least, this makes sense to me. But I'm no microeconomist. Perhaps we have one on LW who can state this more clearly (or who can correct any mistakes I've made).