Users are wrongly picking "Public domain work" for obviously copyright images
Flickr provides two very similar options: "Public domain work" and "Public Domain Dedication (CC0)". The former is for works that are already in the public domain, such as a scan of an out-of-copyright photo. The latter is where photographers can make statement that they are releasing their work into the public domain. Furthermore, in countries that do not permit people to make sucha a release, it acts as a free copyright licence with no restrictions on use and resuse.
By picking the first tag for their photos, the legal value of such a tag is worthless. It indicates the user did not know what they were doing. This can prevent the image being reused or uploaded e.g. to Wikimedia Commons.
Please can Flickr improve the UI to help users pick the correct tag. It should be made clear that the first tag is only for works already in the public domain. Additionally, Flickr could scan the EXIF of any JPG tagged this way, and warn the user it is likely to be an incorrect choice if the EXIF indicates the image is a digital photograph.