Yes, but with so many variables that are out there, could it always be trusted that the employer will play fair with the employee's or the employee's with the employer in a one on one situation, and if not is this why Unions were created for the workers and also the employers in some and/or in many cases ? Once Unions were established for the workers and for the employers, isn't this where some great benefits and protections for all involved came from, like the 40 hour work week, safety in the work place, vacation time, breaks and etc. ?? Were the unions good for the employers also, I mean if the employer embraced the union ?
Unions, as a matter of policy, fight to keep people hired regardless. There was a situation where one employee was chronically negligent in his position, the company couldn't fire him because of unions - even after the guys negligence killed another employee, a fellow union member. The oil company basically stuffed the 'offending' guy into a BS position (where he couldn't do any harm) and have to continue paying him even though the guy is useless to them as a company. -- Agreeably that's an exceptional case, but the point here is that the unions are not working for an individual worker himself per say, they are working for a general cause.
That said, I do not believe unions "started" the 40h work week. If I remember right it was more a universal call from employees in general who, for example, didn't want to work 10-5's and what have you, been around since the like 1800s if I remember. France even had a slogan: "8 hours work, 8 hours play, 8 hours sleep" because they felt that was the best division of the day (which works out to, you guessed it, 40 hours in the old standard M-F week.) Further on though, sometime in the 1930's the US gov. passed the SFLA to mandate OT for over 8h/day and 40h/week and businesses had no choice but to adapt to the new law.
Unions typically fight for things like work place safety, pay rate, and benefits on TOP of the 40h week because that was the cry for fairness in past generations. I'm not so sure they would be keen to attempt any alteration the 40h week standard because a) its what the employee wanted historically, and b) it's kind of dictated by the US gov and therefore rather non negotiable for an employer. I'm not too sure that unions would be to keen on taking up a voice for any schedule that involved mandatory OT as, at least in so much as the ones I've dealt with, they have historically fought against it. Though it's entirely possible that might just be specific to unions where I am because of our geographical location.
Replace the word Union in the embolded above with "Government", now what's the difference in the read ? There is none...
Nominally, unions don't have the power to legislate and enforce laws. I say "nominally", because in the corporatist era, that essentially what they've been granted. They've been bought off by perks and privilege and now are primarily defending their own power rather than the rights of workers in general.