Email is not inventory to be sold, or even depletable. This is more like putting up a flyer in the break room. Technically the cork board gets damaged by the push pin, but it's not particularly relevant.
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.