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Credit cards have always been a true "You better know what you are doing" form of money exchange. The companies literally know what the default rate will be (we will call it X%), and take a view that it is less expensive to write off X% than to spend the necessary man-hours to collect the percentage of x% they could actually get (collect) by trying to work through the situation you (well, your sister) are in.
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And at times they are completely without conscience about who they will give a card to. Even someone who has an abysmal credit rating and a proven track record of not using one responsibly. For this and the previous stated reasons, I can't feel anything but that they reap what they sow when they are maligned in the press and government occasionally steps in (at least in Canada) to moderate their practices upon the consumer.