Fly123456
It's all perception and no doubt falls on partisan lines.
Big generalization, but most HK based crew consider that they are paid 13 months of salary for 12 months work; as is (was) the way of HK. They consider the non-payment of 13th month as an 8.3% enforced pay-cut.. pure and simple.
In the UK, Europe, US etc you are paid 52 weeks, 26 fortnights or 12 months salary for 1 years work; that is the way of such countries. At the risk of confusing you, the parallel situation would be that you "volunteered" SLS and then didn't receive your December salary because the company had an unsatisfactory year and then the HK staff suggested that everyone be repaid SLS and you will get your December salary if there is some money left in the pot.... would you be a bit erked?
As an aside, those FO's on B scale based in the UK, or on a salary expressed as percentage of B Scale UK FO, had their salary scale grossed up in 1999 to include 13th month. So if you are on such a payscale, then you effectively receive 13th month (or a large %) every year....... make you feel better?
I have no idea what the should or will do? If they feel motivated to do anything (which I doubt), I imagine they will do an ex-gratia payment of $HK30,000 or repayment of SLS, whichever is the lesser. That will keep the majority happy and provide best bang4bucks.....