Heh, very true. The key distinction is that physical property is scarce, while intellectual "property" isn't. Copyright laws create artificial scarcity in the belief that doing so will lead to more output; in fact that is the only reason such laws are permitted according to the Constitution. It's logically impossible for a retroactive copyright extension to result in increased incentives (well, until time travel is developed), so I fail to see how it's Constitutional.
For the same reason that the right to bear arms should not be abridged, regardless of what a 'militia' actually is. The purpose is to promote the progress of science and useful arts; but the actual power is to secure for limited times the exclusive right to inventions and discoveries. The Constitution says the times must be limited, but that, IMHO, gives Congress wide latitude to decide what those times should be. As long as Congress makes a determination, however stupid or corrupt, that lengthening the span of copyrights aids progress, the Courts, IMHO, should not be second guessing them.
The founders, in other cases, laid down a precise quantity in a Constitutional provision. They could have said in this case 'no longer than 50 years'. The fact they declined to do so means, IMHO, that the courts should likewise decline to constrain Congress.
I'm just reiterating, of course, what Nick Danger has written. I doubt he's claiming proprietorship over 'judicial restraint' either, however eloquently he's espoused it.