Here you are..... http://www.cad.gov.hk/english/flightstandards.html

This is purely a legality issue. The Civil Aviation Directory (HKCAD) sets limits as to how many hours a crew member can be on duty. Even if the crew wants to go on (and trust me, sometimes they want to get home as much as the pax), it would mean a huge fine for the airline and possible disciplinary action to the crew member.
As to why they would even leave with the possibility of a divert/crew change is two-fold:
1) It could literally be a question of a few minutes and by flying faster, getting shortcuts, more favorable winds, they might be able to make up the time.
2) The delay to get a replacement crew to the departure airport, if even available, would be much longer than taking off and positioning a replacement crew to TPE etc.

Actually, I read the decision at the above link. (I'm a litigator, including appeals.) This decision doesn't say whether or not BA was found to have discriminated, although I infer that from the fact that BA appealed. This decision results from BA's appeal of jurisdiction, -- whether the trial-level court had jurisdiction under the law to hear the case involving discrimination claims by a group of HK-based cabin crew. BA's position was that not enough of the cabin crew's work was done in the U.K. to give U.K. court jurisdiction to hear the case. From the case summary,And from the court's decision, itself:

For FTers who are not lawyers, jurisdiction is a threshhold issue in litigation. A court (or other tribunal) must have jurisdiction (legal authority) to hear a case. If it does not, the court never gets to the substance of the matter and the cannot render any decision regarding the substance of the matter. Jurisdiction is most often based on geography (a county court usually hears most cases in that county) or subject matter (a family court usually hears family law cases such as divorce, adoption, etc.).

In BA's appeal of the lower court's decision, it claimed that the lower court did not have jurisdiction because the cabin crew did very little (de minimis) of their job in the U.K. This appellate decision does not address the substance of the cabin crew's claims, and only addresses jurisdiction issues. Actually (from my perspective), a pretty interesting decision. Thanks for posting it, I would not have seen it otherwise.

Sorry - wording problem. I was meant jurisdiction at all time. And thanks for the clarification.

I was meant that other airlines' FAs may get protection based on where the company is incorporated. But not in CX's case.

what do you mean??

Because it is a legal and security issue...not something CX imposes but rather HKSAR government.

In the early days of civil aviation, crew works just like any other employees. Sometimes they are asked to do overtime, and sometimes you get over-zealous young men wanting to do overtime to make more money and gain fly hours quicker. In any case, most government left them alone, and then a few accidents happened, and now we have very strict rules on crew hours.

It is really for safety precautions and reasons, and as much as I will probably be also mad if it happened to me, but knowing that I get the safest way to be home or aboard will generally appease my anger.

Trust me, as much time it costs us, it costs CX a bunch of more money to have to supplement crews.