http://librarycopyright.net/digitalslider/ This neat site provides basic info on whether a work is under copyright protection or is in the public domain...based on the date of publication. It can get complicated but basically works published in the USA after 1989, with or without a © copyright symbol, are protected and copyrighted until 70 years after the death of the author. Works published before 1989 without a © copyright symbol are in the public domain.
Your interpretation of the slider is very misleading. Your interpretation, as I left above, is not accurate. It is not correct to say that everything published before 1989 without a copyright symbol is in the public domain. Nor does the slider show that to be the case.
From my point of view, this tool seems to be very handy. I'm not knowledgeable in the US copyright though.