Is CC-0 assumed as the default license for Plume, or for Fediverse.blog? That's not a user-friendly default. That means that anyone who doesn't explicitly choose a license is effectively giving up their rights to their writing, putting it as close to the public domain as possible. That's a terrible idea!
If no license is specified, then the author should retain all their rights (except what's required to actually publish). They can choose to use a CC license, even CC-0 -- but that should only ever be an explicit choice by the author.