First, Amazon actually allows multiple separate accounts to have the same e-mail address, and will use the password to decide which one you are trying to log in to. Second, using your e-mail address as a potential username (sometimes, one of many possible, as is the case with Facebook or Google, which will let you use any of the e-mail addresses associated with your account or your username to log in) is very different from treating it as your identity, and you can tell the difference when you ask the question "what happens if I lose access to that address, change to a new one, and someone else is assigned the one I was using previously?", which happens to many people using e-mail addresses provided by third parties (such as companies, ISPs, or universities).