You want to look at the Supreme Court case "Eldred v. Ashcroft." Eldred challenged Congress for retroactively extending existing copyrights, for extending the patent protections on existing inventions could not possibly further arts and sciences. They also argued that if Congress had the power to continually extend existing copyrights by N years every N years, the Constitutional power of "for a limited time" had no meaning.

The Supreme Court's decision was a bunch of bullshit around "well, y'know, people live longer these days, and some creators are still alive who expected these to last their whole lives, and golly, coincidentally this really helps giant corporations."