I think to prove you are a real legitimate user, not some hacker. Better than giving up your Social Security number. Most credit card co./banks insure you against fraud anyway, and if yours doesn't, I would say it's time to change financial institutions.
Notice all the people complaining about how much money they lost during the PlayStation Network hack? Yeah because if anybody's money was taken fraudulently their banks were insured against it, and the people got their money back right away.
Honestly credit/debit cards are the safest form of transaction there is. Bank accounts are all mostly insured for around $100,000. Even prepaid Visa & MasterCards have some fraud protection.
So game on and don't worry. Just keep an eye on your account, which is easy to do in the 21st century what with Smartphones and all.
Right, otherwise anyone who got into your amazon account could buy it instantly since it doesn't need to be shipped. It's the same reason you need to put in your cc info if you are shipping to a new address for the first time.