Originally Posted by
SeriouslyLost
I don't thing you're getting what is being said. In your example a credit card is required for the rental part of the transaction, not the driving part. But that's not what you've been saying or what I've been responding to. If the law says X, Y, or Z is required for travel then an airline cannot extend the law and say that they will only accept X or Y. They can, to go back to your example, require a credit card to pay for the ticket because the law is entirely silent on that point.
It's actually exactly the same thing. The airline isn't saying that Canada won't admit you without Z (using your XYZ vs XY example above). The airline is saying that they won't _transport_ you without Z, and, unless the law prohibits them from refusing to transport you without Z, they're entitled to do that. Again, the law/TSA regs say that you need to provide TSA-qualified ID to pass the checkpoint. An airline is free to have different requirements (i.e. note from your third grade teacher) to let you actually board their plane.