"[S]hould" is the wrong word here. Clearly, they don't maintain such backups. This is clear to anyone using their service. They pay for the service anyway, so apparently it's still worth it to them, even without auto-backups.

Would it make sense for Amazon to maintain automatic backups (and potentially charge more for them)? I don't know. It might make business sense, it might not. But their service is apparently popular enough even without it.