Public domain isn't "viral" like copyleft.

If I take something in the public domain and make a derivative work, the original remains in the public domain, and I retain ownership of whatever additions or modifications I created. So I can attach whatever conditions I want to the copying of those additions.

For instance, Disney's "Sleeping Beauty" was protected by copyright when it was released, even though it was based on a centuries-old fairy tale that was in the public domain.