Energy and (wo)man-hours to run an email server infrastructure and all of the security to go along with it.
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.