It’s Amazon’s internal network, and Amazon’s email system, so they have the full right to control what happens on their resources.
At the same time, I find that deleting the an email after it has been sent by the sender unknown to the sender to be a kind of gaslighting, and dystopian. Having a policy to not organize using work resources might be harsh, but at least it is more transparent as in the proverbial line is drawn in the proverbial sand.
Especially in the world we live in right now - email and other work-hosted communications platforms are not a resource provided by work, they are the WORKPLACE.
That said, even if it is detectible, I think that employees should be allowed to drain some small amount of a company's resources to organize. I am allowed to use company resources for emails like "who wants to go for beers after work?" why should I not be allowed to send "who wants to go organize collective action after work?"