Yeah, United is pretty crappy company and Air Canada can be too.
In December, my wife and oldest son (7), were flying Ottawa - Toronto - Winnipeg. The flight out of Ottawa was delayed but still in time to make the Toronto connection. Air Canada gave away their seats to standby customers over an hour before the second flight was scheduled to depart. AC explained that the itineraries were not connected (Ottawa to Toronto and Toronto to Winnipeg were separate), even though they were booked as a continuous flight.
The kicker, was that they were using the same plane, so there was no way that the flight from Toronto to Winnipeg could leave without the first flight arriving.
Then, AC said they would have to wait on standby themselves to see if anything would open. When two seats did open, they were going to seat them at separate ends of the airplane. My seven year-old son was understandably upset.