You know, I've sorta long suspected that irrops (weather) makes the customer service people feel like they can do whatever the heck they want to get rid of people, since the problems they are trying to solve can be blamed on forces of nature.
Having said that, there really is no reason to ship people to the Anaheim Marriott when there are a zillion hotels surrounding the perimeter of LAX. Someone somewhere is not paying attention, just as someone at United Ops was not paying attention to the connecting pax to SYD on your flight. I don't run an airline on a day-to-day basis, but I can't see the harm in holding a flight to accomodate 25 passengers who had a ticket for it...unless they had oversold by 25, and someone saw that as an opportunity to fill the plane with standbys?
What a lousy experience - here's hoping it gets better down the road.