My experience is that the vast majority of software engineering jobs I see posted require full-time work.
What I think would be helpful is getting away from the notion that working 40 hours a week is somehow 'real work' and anything less is for students and housewives or single mothers who can't work more hours. That's a fact that's also reflected in less-than-proportionate pay which you get with may part-time jobs.
So yes, while it may not be illegal to work part time, it's just not a realistic option for many people.
You have the option to negotiate fewer hours but in a lot of places you will go nowhere. I have several times tried to negotiate eight weeks vacation against a pay cut but the response was always complete confusion or something about policy. I guess if you are sought after it may work but for most commodity devs there is not much to negotiate.
It is difficult to work part time and get benefits such as health insurance. Since this is a necessity for most people there is no practical option to work part time.
To emphasize the contrast to the parent comment on libertarianism and free market. Government does not dictate that a company can not pay for benefits such as health insurance for part time workers.
Insurance providers sometimes have minimum numbers of hours required for an employee to be eligible to participate. I have an arrangement with my employer to work only 4 days a week (32 hours), but I was told that going any lower than that would complicate things with regard to benefits. No, government does not "dictate" necessarily, but the current situation in the US with employer-provided healthcare greatly complicates working part-time, to the point that it's not really worthwhile for a business to spend time figuring it out when there are plenty of people willing to work full time.
If I'm understanding you correctly, you're saying, 'How does the libertarian argument (removing regulation) improve a worker's ability to negotiate.' Is that correct?
So we HAVE the option to negotiate fewer hours, no?