Earlier quoted context omitted.
It is possible and commonplace in jobs where you physically work. Construction, any of the trades, factory work, food service... if you work 40 hours you have actually worked very close to 40 hours. There isn't any real opportunity to slack off or stand around in those jobs. Office jobs, yeah. Many of those are bullshit jobs.
It seems that the answer should be, at least in the shot term, for good managers to recognize the 40 hours "available" concept and provide more flexible schedules. No one gains from having an office full of workers surfing Facebook on friday afternoon. Let people be someplace else if it their queue is empty. From the outside, it seems like startups do this better than most.
As someone on the outside, hearing horror stories about mandatory 14 hour days and regularly working nights and weekends for startups I'd think they were doing it much worse. It's one of the things that has steered me away from working for a recent startup because I refuse to work more than 8 hours a week.
I had an interview at a startup where I asked the interviewer to take me through my average day at the company. He basically said I'd arrive at 8:30 and leave at 6:30 or 7, sometimes later. I told him that to maintain a healthy work/life balance I would be leaving at 5pm every day and asked him his opinion on that. He said it would be technically acceptable since they can't force you to work more than 8 hours, but would be heavily looked down upon. I didn't take the job.