They're really not. There is nothing unique about a tweet or a facebook post. It's just web content and neither facebook nor twitter have a monopoly on web content.
There's nothing stopping you from hosting your own content a la Alex Jones who got 'deplatformed' on twitter for stepping way over the line post-Sandy Hook. His infowars website, btw, is still up.
ISPs terminating accounts for wrongthink, or Google scrubbing one political party's positions from search results, or ICANN blackholing requests for domains like "infowars.com" is a much more reasonable place to draw the line. Because that is what real deplatforming looks like: Functionally erasing someone from the internet. Facebook saying you can't host your content on their data-mining-platform-that-happens-to-look-like-a-social-media-hub is not the same thing.