There is a major problem with the Anti-Counterfeiting Trade Agreement (ACTA) that has little to do with IP or the internet: how does international law get made—by the President alone, or with Congress's involvement? ACTA's key problem in the United States is a Constitutional question that turns on the separation of powers. The President, or an office of the executive branch like USTR, can negotiate treaties that fall within presidential powers. But for topics that fall within Congressional powers, like IP law, the Constitution requires that Congress be involved in the process.
The most obvious and difficult way to involve Congress is through Article II of the Constitution. Under Article II, a treaty negotiated by the executive branch is presented to the Senate for ratification. The process is notoriously difficult, because it requires two-thirds of the Senate to approve. So USTR, almost understandably, wants to avoid the Article II process if at all possible.
A number of years ago, this wouldn't have been a problem, or at least not a Constitutional one. Congress gave USTR "fast track" authority to negotiate trade agreements, subject to an up-or-down vote at the end of the negotiating process. This authority, however, expired in July 2007. ACTA wasn't announced until October of that same year. Fast track wasn't great, because it didn't allow for amendments, but at least it allowed final oversight over the executive branch by Congress. It also allowed international law to be made, because the hurdle of Senate ratification for Article II treaties can make that process come to a standstill."