Surely the big issue here is whether or not the individual was advised that the plane would make a transit stop in the US. This responsibility should (IMO) lie with the provider, be that the agent or airline. If you buy a ticket on a Canadian airline to fly from Oz to Canada, it's by no means rational to assume you'll need a US visa.

I think this was handled badly by the airport staff. A friend of mine (Hungarian passport) hit the same issue some years ago, flying AC from LHR to SYD via YVR. On check-in at YVR, the visa issue surfaced. AC admitted liability as this should have been picked up in London and routed them via HKG at no additional cost. However they then had to apply for US visas for the return leg, costing far more than they saved by taking a trans-atlantic/trans-pacific route in the first place.