Actually, the law HAS changed. Originally, copyright was for a much shorter time (14 years, renewable for another 14 years if the creator was still alive), but in modern times the length has been pushed so far that the "for limited times" part of the constitutional clause that gives the U.S. government the authority to CREATE a copyright law is, for all intents and purposes, irrelevant. Anything you see created today will still be copyrighted long after you die.
Unfortunately, this makes the public domain a nearly worthless concept; copyright is limited so that things will eventually become public domain, but with copyrights so long, nothing relevant to modern society belongs in it. Hell, we have entire forms of media that will never have a single item enter public domain until you are dead, buried, and dust.