You completely missed every point being made here.
This is obvious to most people, but not all, so getting this out of the way first. Some jobs are all about having a body covering specific hours. We're not talking about those jobs.
We're talking about the type of jobs that are about getting the work done. The exact hours worked are irrelevant in these jobs, as long as the work is done on time. Most office jobs fall into this category. A large portion of these jobs are salary, not hourly, specifically because we acknowledge that the job is based around getting the tasks done, not around the hours worked.
Most people don't have 40 hours of productivity in them in a week. Those that do generally don't pull it off in the long run, just in ideal scenarios or crunch bursts. There's a ton of wasted time in the work day for most people.
There's been a ton of studies that show reducing hours makes people more productive and happier. It's really, really easy to measure if it's working or not. These jobs were always evaluated in terms of "Is the work getting done correctly and on time?" Your metrics don't change at all if your reduce the hours. If things are still getting done, it's a success. If they're not, it's a failure.
As to your concept of fairness - why is it fair that the salaried workers are generally expected to stay past 40 hours when necessary, but aren't allowed to leave early when 40 hours aren't necessary?
And if you paid any attention to the article posted, you'd notice cutting hours *saved* taxpayer money. The workers accepted a reduction in hours instead of a raise. The work still got done on time. Workers were happier. It cost less than keeping the workers at 40 hours. Everyone won.
All of this is really, really obvious to basically anyone that's ever worked an office job.