In any case, whatever the reasons, it's a very strong signal. Maybe it's happened, but I don't remember a legitimate example with a shortened amazon url redirecting with an affiliate id, while I've seen lots of this kind of spam, so I'm pretty okay with people using it as a sign they should be suspicious and flag if in doubt.
@Cerberus I doubt spammers are looking carefully enough to know what stackexchange does, but it's certainly a solid general practice for affiliates, gives you a clean un-suspicious link.