The idea that someone does this willingly implies that the user knows the implications of their choice, which most of the Fediverse doesn't seem to do (see: people asking questions like "how do I delete comments on a server I've been defederated from", or surprised after finding out that their likes/boosts are inherently public).
If the implied license was enough, Facebook and all the other companies wouldn't put these disclaimers in their terms of service. This isn't true in every jurisdiction, but it does apply to many important ones.
I agree that international copyright law should work like you imply, but on the other hand, this is exactly why Creative Commons was created: stuff posted on the internet can be downloaded just fine, but rehosting it is not allowed by default.
This is also why I appreciate the people who put those Creative Commons licenses on their comments; they're effectively useless against AI, which is what they seem to be trying to combat, but they do provide rights that would otherwise be unavailable.
Just like with privacy laws and data hosting laws, I don't think the fediverse cares. I think the fediverse is full of a sort of wilful ignorance about internet law, mostly because the Fediverse is a just a bunch of enthusiastic nerds. No Fediverse server (except for Threads, maybe) has a Data Protection Officer despite sites like lemmy.world legally requiring one if they'd cared about the law, very little Fediverse software seems to provide DMCA links by default, and I don't think any server is complying with the Chinese, Russian, and European "only store citizen's data in locally hosted servers" laws at all.