The basic idea behind office hours is that if you can't make people work, you can at least prevent them from having fun. If employees have to be in the building a certain number of hours a day, and are forbidden to do non-work things while there, then they must be working. In theory. In practice they spend a lot of their time in a no-man's land, where they're neither working nor having fun.
http://www.paulgraham.com/opensource.html "Workplaces"