Most of the text of laws I have seen are for the state, not the federal government, but they read much more like your latter example (e.g., "the text of Article 8 shall be superseded by..."). In any event, your argument that the acts are not really increasing the size of the actual law doesn't ring true to me. For example, the federal law by which I am governed consists of 18 volumes, each 1300-1500 pages in length (as of 2006). Supplements are issued until the next release in 2012. You might be able to read 23000+ pages but could you keep it all in mind every day as you live your life? Now add in the state laws for your home state. And those you visit. And any foreign countries you might care to see. However we got here, the end result is unknowable by any single person much less every average citizen.
As for whether this particular law is needed, I would argue that the Computer Fraud and Abuse Act, 18 U.S.C. 1030 (the "CFAA") which was passed in 1984 (Hello, Orwell!) covers most if not all of the acts the "Informed P2P" law covers. As others have pointed out, its true intent is most likely to criminalize all peer-to-peer communication to make some group's life simpler (e.g., the MPAA/RIAA or government intelligence services) at the expense of everyone who uses peer-to-peer software for legitimate purposes.