ACTA could be signed in the U.S. without Senate ratification because it doesn't change a single thing in U.S. domestic IP law, so there really isn't anything for the Senate to ratify. Wyden knows this; his request is largely grandstanding.
This is not necessarily true in other countries, which is why their legislators are getting involved.
Technically, you're somewhat wrong about other countries. For instance, in France, even if ACTA had no impact on the legislation, it would still require ratification by a law. Because, the list of treaties that require ratification is so broad in France (Art. 53 of the Constitution), that almost only symbolic agreements are exempted of ratification by the parliament (or referendum). And that's the case in most of European countries.
This is not necessarily true in other countries, which is why their legislators are getting involved.