The signing part is obvious, but posting to public relays is basically shouting in a public forum. You can always claim your content, but others can claim it as well. I am wondering if we all assume similar things or do we differ in our expectations? For instance, copying content is 0 cost. We might agree that impersonation is unwelcome and discouraged or not. Do users, clients or relays care? If we have a licence tag on a post, does it mean anything in practice?
Discussion
Too true on the copy and stealing part. That's the whole internet though. I'd presume it mimics the real world, in that your copyright ownerships are only relevant to the extent that you can enforce it legally and/or shame the thief. That part is a puzzle that's 1000's of years old. Get philosophical pretty quickly.