But as you say atlas, the crucial difference is one is obvious to you (acute fatigue, short term, feeling tired, obvious when you shouldn't fly) and chronic fatigue (feel maybe a bit fed up, irritable, no obvious stand out red flags). But with chronic fatigue, when the workload ramps up, fixation occurs, things get missed and the wrong decision is likely.
It is no good airlines saying 'pilots should not report if unfit' when the schedules drive people into chronic fatigue scenarios a pilot may feel in all honesty they they are not at their peak, but otherwise OK. This is why we have FTLs. This is what SMS is meant to prevent. To prevent company induced chronic fatigue.
If the only real line of defence is an individual crew member's judgement as to whether they are fit to fly, then the airline in question is more concerned with how blame can be effectively apportioned to the crew and has no real interest in safety. That much is clear.