I think that this may be actually down to Australian Government Restrictions. The same happens on Qantas flights from Dubai to Australia - after the boarding pass check there is a secondary bag check just before boarding and any liquids over 100 ml, including water bottles, have to be surrendered. Even in QF business class, so definitely not a money making-issue. This only is the case on flights TO Australia. If you have issues on the return flight back to KUL (if you have one) then yes, Air Asia is being mean
Shuttle