For a while now, account thieves have been putting authenticators on their stolen accounts to buy more time for their scumbaggery. Blizzard has recently made that more difficult by requiring email confirmation when an authenticator is added to a Battle.net account. Rather than just logging in and putting in the appropriate information, you now have to follow the steps in a confirmation email sent to the address registered in your Battle.net account.
Note: Changing the email address on the account requires not only your password (which the account thieves already have at this point) but also the answer to your security question. So make sure the answer to your security question is not guessable or obtainable by any phishing information. As I have suggested before, if you use a password for your security answer rather than an actual answer, you are adding a very thick level of security. Make it a separate password you use just for security questions, like p45sw0rd (don't use that one).
Of course, the best way to prevent someone from stealing your account and then adding an authenticator to it is to put an authenticator on it yourself. There are keyfob and mobile versions available.
[Thanks for the tip, Joel!]