So, I looked up the SMTP RFC, and yeah, the "local-part" (as it is determined) is to be treated as opaque by everyone BUT the domain in the address. Meaning that everyone must treat the addresses differently regardless of how GMail or anyone else interprets the semantics...
AND THEN, it turns out that while things are required to be case-insensitive, things are ALSO required to be case-sensitive. Basically, no one should ever assume that the local-part of the email address can be treated as caseless.
So, there you go, if Amazon doesn't let you sign up as both smith@example.com and Smith@example.com, then they're totally out of spec...
But to the deeper part, why would Amazon not disable an account when someone with a local-part semantic collision calls in to object to getting the emails? "These two addresses are treated as semantically identical by my email provider, please figure out how to fix the other person's account," doesn't seem like a horribly unreasonable request... I'm sure they'd do it for Smith@example.com coming from smith@example.com...
Bitching about the RFCs and complaining that GMail is the problem is entirely misreading the RFC, and misreading reality in fact...