It's generally recommended to place a Back, or Cancel button depending on information committed, on multi-step systems to allow the user to escape a process.
It is one of the most basic heuristics for interaction design to support user control and freedom by allowing users an "emergency exit" out of any situation they may have entered.
Cancel is mainly useful for multi-step dialogs where the user has progressed past one or more pages with actions. At this time, pressing the Back button will not undo these actions and it would be better if the user would click Cancel.
However, sites like Amazon do not conform to this recommendation as seen in both their checkout and registration process. They do not offer a visible back or cancel on either their checkout or registration.
What is the rationale for not offering a user a way to escape these processes? Are there any studies, articles about the benefits of removing these feature from checkout, registrations or other multi-step processes?