If I am uploading historical photographs that are in the public domain, I should absolutely have an easy way to mark them as such. Not everyone uses Flickr like Instagram to share the last neat thing they saw on the street.
I get that you're concerned that people don't understand the public domain, but I think the response to that is more (and better) explanations about what it is and why it's important, not disallowing people access to tools that they want on a platform they use.