Different folks are paid for different work. Many people are paid by the hour, so it is reasonable to expect that every minute of every hour they're supposed to be working that they would indeed be working.
Many other people are paid on salary. They're expected to get 40 hours worth of work done each week (for instance), and are evaluated based on that. If they have to spend 50 hours to do it, or as is more often the case, have to spend 60 hours to accomplish 50 hours of work (because there aren't enough people employed to ensure everyone's work-load is commensurate with 40 hours of work each week), then that's what they do, and without additional compensation.
In that scenario, what a person is doing in any random minute during the day has no impact on the expectation with regard to how much work they're supposed to be doing, and so the relationship between the fact that they're at work and whether they're working or not is less relevant.
Some employers actually capitalize on this. My wife works for a company that used to feed its employees dinner if they decide to work late, and still have many such amenities intended to foster a homey environment at work (including a big-screen HDTV and Tivo in the lounge!) They do this because the more those employees feel at home at work, the more high-quality work they get out of these folks. They don't begrudge their employees anything that makes them feel more comfortable working the long-and-hard hours they work. Those companies realize they get a very high return-on-investment from every minute of unwind-time their employees take.
Even my own employer shows some signs of this. I am often forbidden from working, even while on company time. I didn't sleep one night last week, and so I sent a note into the team that I'd be working at home (because I was a little concerned about driving to work). My boss replied, in no uncertain terms, that my job that day was to try to sleep, and that he'd call me if there was something they seriously needed.
So, in the end, I think the answer to this issue is that, "It depends." For many people, their time is what their employer is buying from them, and surely just as you have the right to expect to get 12 eggs when you buy a dozen, the employer has the right to expect to to get 60 minutes when they buy an hour from an employee. For many other people, their industry is what the employer is buying from them, and the actual minutes and hours are secondary considerations to the work they actually accomplish.