The reasons are: 1. People who don't like you won't be able to subscribe you to (legitimate) stuff you don't want, therefore flooding your inbox with legitimate e-mails that won't be blocked by SPAM filters. 2. You are making sure you didn't make a typo in your e-mail in case you need to reset your password, so that somebody with similar address doesn't get your password. 3. Obvious marketing legal reasons (double opt-in, i.e. you did confirm that you want to receive marketing spam from us).

The "easy" signup isn't with username-password-email-submit, it's with OpenID.