Euro
Probably because she provides the goods or service for which she charges in a reasonably short time scale - thus limiting the Credit Card Company's exposure. With an Airline the flight could be upto a year in the future so you can see why the Credit Companies don't want to be "on risk" for 000,000's of pounds worth of exposure for such long periods. The cash is released to the Airline as "flown revenue" once the flight is operated (contract fulfilled).
That's the theory anyway. What happened with GSM / XL remains to be seen.