I wonder why only the US. If they believe pay transparency is the right way forward, they should do it globally. Surely there's no legal hurdles in posting pay ranges for any country.
As with all negotiations, once you've stated a number you've put a lower (or upper, depending on the side you're on) bound.
As you mention, if they really wanted to be transparent, it would be global, but I doubt they want to expose those differences.