There's a difference between being in public and being available for public use. Books in a book shop are "in public", but only those out of copyright are available for public use. Similarly, blog posts are in public, but not being given away for anyone to resell or include in their product.
For an image/photography example, I may have my photo taken at an event and sign a release or agree to the event's T&Cs that say they can put publicity photos on their social media or website, but I don't intend that my photo will be used by an unrelated party to generate images of my face (this may not actually be restricted, yet, but I'd prefer it to be and I suspect few would consider it a reasonable outcome).