A quick answer to this is that the newspapers themselves are in the public domain, but neither the microfilm or the digitized versions of the newspapers are in the public domain. The microfilm is owned by the entity that paid to have it filmed (sometimes History Colorado, Denver Public, an individual library, or a local historical society). The rights to the electronic files are controlled by the entity that put them online, the Colorado Historic Newspapers Collection.
This is similar to when electronic copies of historic photographs are made available online in a library’s collection – for example the Western History Collection at Denver Public. The photograph is available for research purposes, but the rights to the use of that photograph remain with the entity that digitized it (in that case, Denver Public). The reproduction rights of photographs for a library may be what funds the digitization of the photographs, so library may maintain the reproduction and use rights for that fund-raising purpose.