This post has been a little US biased with a flavour of Italy.
Here in the UK my company's regs put the responsibility on me, stating that I am provided with adequate provision for rest before a flight.
In fact this is often untrue with approx 24 hours between flights which makes proper rest impossible to arrange.
If I were to call in "fatigued" I'm sure I would be seeking a new job.
In my experience we just get used to preparing ourselves to work hours normal people would be horrified by.