I'm arguing that it's pretty preposterous for a global decentralized federated social platform to assume some authority (enforced by whom, that'd require a central group of enforcers) over the licensing of posts on the network by invoking US copyright law. Conceptually it doesn't make any sense. So the only thing Mastodon can do is at a protocol/code level require that public content flowing through the pipes be licensed for public consumption (copying and distribution) on the network, the colloquial "public domain".
> It would be easy enough
I can't really imagine the matrix of which nodes can federate with which others based on the default content license they impose on posted content. "Oh you can only view my post if you use this server over here because it uses a compatible downstream licensing configuration." Would that really "work"?