I would think that subscribing to a community could be coupled to a license. Servers do not randomly send data, they only send it to other servers that are subscribed. And a server could technically decline a subscription.

But anyways, by default, copyright is with the creator. No idea what that looks like in legislations around the world, but if I remember correctly, in EU, just because you give a copy of a e.g. song you wrote to someone, does not actually mean they can do with it what they want. By default, you have all the rights, and the someone else needs to grant them to you. So if you give that someone also a contract where it states that he can play it in front of an audience, then they can, otherwise they cannot.

However, I am not sure how much implied consent can play a role here. By posting something on a fediverse instance, since the purpose of the fediverse is to share these posts with other servers, then by posting you may implicitly agree to this data being shared, and the next server can share it with another server again, and so on. This is the basic “boost” functionality of mastodon.

I believe though that because the purpose of the fediverse is not explicitly to train AI models or to sell the posts to someone else, it may be illegal to scrape all posts off to feed e.g. an AI model. But may also not be. We will never know until someone starts doing it and someone else sues them.