You can see this "default to public" mindset in the way mastodon does not allow you to set who gets to reply to you or let you address a post to just your local instance (outside of @darius's Hometown fork). The assumption is that you are always available to anyone, like email but with a public address.

For all the community talk on mastodon, it does not contain many of the required elements of real community building, like creating closed groups.