I'd argue that isn't settled yet. Take this. I run my own server, I don't want your "licensed" comment. I can go and add that to my ToS right now, that anything you give me will be trained. I say in there that I will disregard any license on there, and by placing anything on my server you are relieving yourself of any license. Since it's my server, I can say that. "If you don't want your data trained on, don't put it on my server."
So, I don't think it's as simple as you make it out to be. It's exactly the same as Facebook's ToS, they state in there that by using their service and putting data on their servers that you allow facebook to use that data however they see fit. Why is me doing the same thing on my server any different?
So, to use your own format
“Other server owners, did you follow the license that the content is licensed with?”
"No, you're honor, you can clearly see on our homepage that it's stated that any and all licenses are lost when they give us information.
Did they accept any ToS saying as such?
"That's irrelevant, our legal terms are clearly stated on our website, if they didn't want their information shared to us, then they shouldn't have shared them with us."
This format isn't well designated in the courts, there are no real precedents for the fediverse or how it works. You're arguing that it should work that way. I'm arguing that how it should work is irrelevant, and right now there is nothing stopping anyone from using unencrypted data given to their server in any way.
What could happen, but isn't really set up at least on Lemmy yet, is that when one server federates with another the receiving server sends a ToS/license that is server wide, forcing the subscribing server to accept or to not federate. I think that would shore up gaps in the law here, because in your example they could respond with "Your honor, we gave them terms and licenses to subscribe to our updates, and they accepted". I also think that then would be required for new users signing up, to see how data is licensed from other servers. If this were a github issue I'd back it 100%.
However, in both scenarios, both current and what I'd like to see, I don't see that adding a license at the bottom of your comment will ever hold up in court.
(Of course I'm not actually doing that, this is all a thought exercise, but I do 100% guarantee someone is just accepting all of our data and using it, license linked or not)