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.
So they need to nut up and shut up. If you can't be trusted to let your laborer's organize in good faith, they have no ground on which to stand when the bill comes for a second labor specific communication network comes due.
All that waste they could avoid by just being decent disinterested network operators. Sad really.