Amazon's been in the business long enough to know the importance of data security. But for argument's sake, let's say that your Amazon account name and password are transmitted over the air in cleartext by the Kindle. Now what? You're a bad guy, you used Aircrack-ng, and now you've got access to an Amazon account. But you can't get the credit card number out of the Amazon account, just the last four digits. So you can't buy stuff on Netflix, or from the random web site mentioned by the OP.