There are people whose jobs are bad deals, and one of the ways a job can be a bad deal is that it consumes too much of your time. A reasonable effort to protect against this is to impose laws against jobs that consume too much time, in order to protect people's quality of life. France has gone particularly far in that direction with its 35-hour limit on the work week.
Such laws do not work by simply improving everyone's job whose work level is beyond the threshold. That's impossible in a free society. Instead, what they do is make the above-threshold jobs illegal. Continuing to work at them then becomes a crime, and over-threshold people instantly lose their jobs. The hope, often fulfilled, is that those jobs are replaced by new ones that have lower hours with, perhaps, lower pay. The dark side, though, is that some of those jobs don't come back at all. Further, some businesses collapse entirely.
An poignant example to consider is startup companies. Paul Graham has a great blog, largely about about computer startups, and he writes of them:
Economically, you can think of a startup as a way to compress your whole working life into a few years. Instead of working at a low intensity for forty years, you work as hard as you possibly can for four. This pays especially well in technology, where you earn a premium for working fast.
The question I am left with is: should these guys be allowed to work horrendous hours? Workers at startups voluntarily choose to work at horrendous intensity because they want a chance at huge payoffs.
I think yes, surely, these guys should be allowed to choose long hours if they want. They want that life, and they are providing a tremendous good to the rest of us. Disallowing it would make them grumpy and the rest of us poorer.
If one says yes, here, however, how can one ever draw the line about what jobs are allowed to have long hours and what ones not? It seems that if one says yes here, then one says yes always: you can always work long hours if you want. (I find it pleasing when put this way. Whose business is it but mine how hard I work?)
Nevertheless, there is still room for protective legislation here, even if we do not impose an outright ban. We could follow the lead of questionable medical procedures: require people to sign extra documents before they agree to long-hour jobs. The required document would read something like, "I have been informed that this job requires more effort than the U.S. Department of Good Living recommends." This way, people can choose however they wish, and they can avoid being tricked by the fine print into working too long.
This approach makes sense in a free society. It adds paperwork, and it adds administrative and enforcement officials, but it leaves the ultimate choices up to the people involved. Personally, I find this approach distasteful--I can read a contract, and I think my countrymen had better learn to, as well. Nevertheless, if we must have the nanny legislation, let's do it in a sensible way. The approach in the last paragraph accomplishes the main achievable goal--avoiding tricks--while ultimately leaving individuals free to choose how they live their own life.