As I recall, this was an early decision by Amazon during the days of dialup accounts and limited email access. Two different people (eg, a couple) might share the same email address but want separate shopping accounts.
[Edit] This was covered in RISKS in 2008. See http://catless.ncl.ac.uk/Risks/25.39.html#subj12 .
> Steve Loughran: Regarding the issue about Amazon allowing >1 login per e-mail address, its a historical legacy that they probably hate. Remember back in 1995 when the whole family had one compuserve or AOL e-mail address? That's when Amazon was created, and that is where they came up with the fact that an Amazon user does not have a 1:1 mapping of e-mail->userID. What they do have is a mapping
of (e-mail,password)->userID; you can create two accounts with the same e-mail address, but you will get into trouble if you try and give them the same password. I'm not sure what happens, so try it and see.
> The newer Amazon services, such as the Amazon Web Services, have a stricter "one e-mail address" per account rule. Clearly their support organisation has learned the error of the original design decision.
It doesn't seem possible merge multiple accounts. See this Amazon transcript for a recent example: http://www.amazon.com/forum/amazon?_encoding=UTF8&cdForum=Fx...