The 40 hour week is already legislated. If you work over 40 you are supposed to get overtime. Overtime needs to be requested, though - that's an important point. If you're going over 40 you're supposed to tell your boss to get permission, and the boss is supposed to say, "nah, go home."
I know it doesn't work that way.
Its because as programmers we never work 40 hours productively. Quite a but of our time goes in procrastinating, reading stuff on sites like HN, Facebooking etc. There fore there is almost an untold understanding that 40 hours is not to be taken literally.
In other words, "the code's compiling" explanation for idling covers the linking up of modules in the code and in our brains' mental models of it.