Amazon has offered them subdomains based on their country code, so .br.amazon for example. But they want to share access on the root domain, but that's not really how domain name systems were designed. They are designed to be hierarchical, so it makes sense for Amazon to grant full control of a subdomain to them rather than share.

Sharing would be a security nightmare also. Imagine any cookies set for .amazon ? How would the browser share between amazon subdomains vs non if they shared the root?