Why is Air Canada refusing to honor the US policy to enable Americans?
Because their company policy is that all passengers have valid travel documents. The US has no authority over private businesses' policy and it certainly has no authority over private Canadian businesses' policy. If someone does arrive at a US port of entry and the US does not grant them entry, the airline is responsible for removing them from the US. This is why airlines require that everyone's travel documents are in order before they board a plane. There is nothing surprising about an airline not letting someone board a plane without proper travel documents. I doubt it is just Air Canada that is not allowing passengers to board with expired passports.