Found this excellent graphic, which tells a story in very few words – the expansion of United States copyright laws over time – on Wikimedia Commons. It was created by Tom W. Bell in connection with his article   Escape From Copyright: Market Success vs. Statutory Failure in the Protection of Expressive Works, I think he makes a persuasive case – but whether or not you agree with his conclusions, this particular point is clear and concise because of Mr. Bell’s infographic.