How does retroactively adding copyright to public domain works in anyway "promote" it's already been accomplished. This the same argument that was used Eldred vs. Ashcroft where this argument also failed. It was a bad ruling then and it's a bad ruling now.
If we want to have this decided by the lawmakers then we should amend the Constitution to grant this power.
Of course adding a given public domain work back into copyright doesn't promote the creation of any specific work, but harmonizing American copyright law with European laws, generally, might arguably promote creation of works.
The key phrase in the clause is that:
[The Congress shall have Power]* to... [secure] for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries.
*) Article I, Section 8.
Like the other enumerated powers, the language of the Constitution gives Congress a lot of leeway in exercising that specific power.
If you ignore the fact this is still under copyright overseas, you are setting the precedent that they can do the same in return. No?
Everything should be immediately copyrighted if you mark it and specify its license terms.
14 years is long enough for total copyright but attribution rights should last forever.