Most of the time, Amazon considers you to have a "soft" login. Want to view your order history? Password please. Change an order? Password. View/update addresses/credit cards? Password. This is sometimes irritating to the user, especially when clicking the "where's my stuff" link from an Amazon order email, but it serves Amazon well in that they can trace most site actions to a user account while at the same time maintaining an acceptable level of account security.
Sometimes (I believe), you will even get a password prompt for a "one click" purchase. Even if you don't, your one click purchases can only be delivered to a previously configured address, and to add a new address requires entering the credit card number again, and, of course, your password.
I find it less intrusive than Linkedin's soft login, which really annoys me, especially as I cant remember my password often. You can go as far as writing someone a message, then you need the password. If I can read my private messages, surely I am good to send one?
Sometimes (I believe), you will even get a password prompt for a "one click" purchase. Even if you don't, your one click purchases can only be delivered to a previously configured address, and to add a new address requires entering the credit card number again, and, of course, your password.