It's really annoying that some airlines do this -- it really negates the benefit to the consumer of an "electronic" ticket: Instead of keeping track of a paper document for fear of having to repurchase, which you will presumably at least be able to store safely in your home until it's time to travel, you have to ensure that the card that you carry around every day isn't lost/stolen/replaced in the interim.

There really should be a better way to validate that a given credit card number is really valid for this customer than this.