Sick is not the same as fatigue. Talk with an aeromedical doctor if you wish, I have both in the UAE and the UK. Any one can call sick but being responsible enough to say one is fatigued demonstrates a responsible individual. The 320 fleet with EY spends most of its time operating thru the night. Each pilots situation is different some have young families and resting can be problamatic, some are older and cope badly with lack of sleep this is made worse by operating almost constant nights. Some of the flights undertaken often involve a long sector and then on some occasions up to 3 hours on the ground until its time to get going again.
If it is as you put it a case of 'abuse' then is it not a 'safety' issue, this was just one example there are many more.