I doubt very much that security rules in the US were the main reason that passengers were kept on board.
The MEL-LAX operation is always going to be critical from a crew duty/delay point of view.
If the crew are presented with a fault that Engineering is unsure of precisely when it will be fixed, then it makes sense to keep everyone on board. Maybe that should be explained to passengers. The 20 hour limit is inviolate.
Every A380 or 744 leaving MEL for LAX has a maximum of five hours to get away after a delay of any description or the same thing will happen. There are no standby crews sitting around for any aircraft away from the Sydney main base. No airline in the world maintains standby crews away from its main crew bases.
If passengers wish to have entire crews on standby at every departure, that are available to cover the occasional technical delay, then fine... stump up the money. If not, perhaps a little more understanding of the complexities of the operation, and a little less faith in the idiots of the media, and one or two posters here might be the order of the day.