With few exceptions, the U.S. Supreme Court has rendered wise copyright decisions consistent with the Copyright Clause. Unfortunately, Eldred v. Ashcroft adds to the exceptions. The difference is that the former are positive law, and the latter natural law, decisions.
L. Ray Patterson,
What's Wrong with Eldred? An Essay on Copyright Jurisprudence
Available at: http://digitalcommons.law.uga.edu/fac_artchop/355