You actually should not because the US has plenty of legal protections that cover the employees right to organise as well as prohibiting companies from engaging in practises that make this impossible.
I'm pretty sure this would exclude utilizing company equipment (i.e. email) for that purpose though. To illustrate: if you worked at Walmart and used their posters to make picket signs that wouldn't fly under employee rights.
But as you're helping shoehorn property rights into support for authoritarianism, I will point you to some legal advice directly:
https://www.nolo.com/legal-encyclopedia/shop-talk-rules-unio...
As long as Amazon prohibits using their calendars for all non-work content (and enforces this), they're in the right. Otherwise, this is censorship targeted at employee organizing and they're in the wrong.