Since many of our books are published in-progress, we want to ensure that readers can get these free updates they are entitled to. So, we automatically create user accounts based on email addresses.
Because we do this, one consequence is that if you buy a second book using the same email address, we do make you login, which can involve a password reset or a sign in token. Otherwise, you would either lose access to the other book(s) or have 2 accounts with the same email address, and neither of those choices are very good.
We could offer a "just give me the current book, I don't want updates" option, but this would involve a lot more complexity at purchase time than we think is helpful.