But the practice has somewhat backfired. Some members of the general public now erroneously believe that any image lacking a watermark is a public works.
For one specific anecdote, a local popular bar lifted an image of their establishment found on Flickr and used it in their marketing materials (posters mostly). When contacted about this by the photographer, the establishment claimed due to the lack of watermark and being their establishment they were well within their rights to use the photograph. They eventually agreed to stop using it as a "gesture of goodwill" (but never paid for it, and weren't sued as the cost of even small claims was likely higher than the damages).
You read stories like this often if you hang out on photography forums. The general public seem to conflate a watermark with copyright and a lack of watermark with a public works. Basic copyright needs to be taught in schools.