Interesting issue. I wonder if it would be possible for a country to set up an extremely long copyright term (with a huge administrative fee) that would have to be respected under US law (and other adopters of the treaty).
I'm sure many music/movie studios and authors would gladly pay $100K to copyright each of their works internationally for 500 years...
I'm sure many music/movie studios and authors would gladly pay $100K to copyright each of their works internationally for 500 years...