Quite a few travel agents have this requirement, and Crystal Travel introduced it about a year ago. You may find the flights through expedia or opodo at a fairly similar price and they won't require that. Occasionally if you book with the airline using a different credit card, the airline will require the traveller to hold a photocopy of your card and passport to prove that its not a fraudulent booking.
As for e-mailing credit card details, I did that to a Qatar Airways service desk in Doha for special meet/greet services on a card I'd never used before, and had fraudulent transactions on it a week later!!! (The bank repayed me them, and despite me suggestion they investigate they said it's not worth the time for them).
