The problem with the wide body jets is that the Aircon as supplied by the APU is very rarely 'man enough' to adequately cool the aircraft to acceptable temperatures.
As the ground air system has the same problem the action of closing the 'curtains' certainly seems a sensible one. The health and safety dictorate has already removed the ability to run engines on stand or open opposing doors at either end of the aircraft.
It would seem that BASSA are merely being obstinate for the sake of being obstinate and ignoring that the comfort achieved would benefit both their members as well as the customer.