Source? While they could certainly do this IP addresses are shared quite frequently due to NAT. It seems inevitable that a banned IP address will login to another account at some point.

I can't imagine they ban IP addresses lightly.

I've seen it happen and also have spoken to people within amazon who have confirmed. They might do it with cookies instead of IP addresses but if you log into a banned account from a computer that regularly logs into a non banned account, your account will be banned, period.