@stefan @BeAware I think there could be a legal exploration into this topic as well.
I know implied copyright has been discussed for years regarding publishing to the web or to an RSS feed. In the US I recall a case where a subscription news-syndication platform was pulling in the full text of RSS feeds and they were being sued under the idea that just because someone posts an RSS feed doesn't mean you can use the contents of that RSS feed for any purpose.
I'm not sure if the same would apply to ActivityPub but I don't know if I have an explanation for why it wouldn't