In both cases, the individual is limited in their choice. Without legal restrictions, the individuals choice is restricted by economic and social constraints. E.g. in low paying jobs, less hours is not an option, because of low pay, and in "high" paying jobs, it isn't because you won't have the job long. Precisely because of the implication you give "more hours -> wanting to work harder". And partly because your employer doesn't want to go through the hassle of having to handle different work schedules, managing more people.
The "European" variant is a collective choice countering the socioeconomic pressure.
In Europe you are "forced" to have a medical insurance, in the US, you are free to choose/lose it with your employment.
I would claim, socioeconomic constraints are limiting your choices much more, than laws do.
Why is this confined to only the people already working?
That is: if I have the opportunity & desire to work on something, you shouldn't interfere with it. But me having the desire to work on something shouldn't require you go provide the opportunity.
But whatever our beliefs, we already have sets of rules, legal and social, which limit people's choices in the job market. The ability to hire people is a privilege which comes with responsibilities, part of which is a responsibility to society. This is why developed countries have minimum wages.
Now as to what extent or whether at all, you can place limitations on employers without placing limitations on employees, now that is an argument worth exploring.
if people want to work 5 days a week, it's because 5 days is the norm, and noone gives much choice.
the norm used to be 6 days, then 5.5 days, and it was a big campaign in germany to get it down to 5.
btw the current norm is actually 35 hours for office work, 7 hours per day, or, if you do 8 hours per day 4.5 days. now they want to shave another 7 hours of that to 28.
all such a regulation does is to define what is the default. it does not dictate that i am only allowed to work those hours. i am still free to negotiate less hours if i want to. negotiating more is harder because of overtime rules but it's possible.
Of course, the option to have a 4-day week should be on the table, but this shouldn't be mandated by decree - but instead, a choice the worker and the employer can negotiate and come to a market driven agreement. Otherwise, you'd be building in market inefficiencies, and this leads to wastage.