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.