Several years ago, I sent transcriptions of documents to a distant relative. She posted them to US GenWeb archives. Her submission contains a footer saying the transcriptions are copyrighted by the submitter and cannot be used, transmitted, etc. by anyone else without permission.
She sent my transcriptions to the appropriate county GenWeb sites. These transcriptions are mine because they include my unintentional errors. I know she didn't get the originals herself and retranscribe them making my exact same mistakes. I'm all for copyright protection, but don't claim copyright to something someone else sends you.
I am all for sharing and do it regularly. But if you take what I share with you, claim it is all yours, and claim copyright to it, we are done sharing.