Sure, there are certain things that can be achieved by technological means. My point is that when it comes to certain areas of NSA activity, you're dealing with more than just simple legislative fixes. The specific example mentioned by Mike Hearn, the NSA tapping into international leased lines, is really illustrative. Our whole government is structured around the assumption that the executive branch is supreme when it comes to activities outside the U.S. To make it illegal,[1] for the NSA to tap into foreign leased lines would require more than just legislation, it would require Congress to decide to regulate the NSA's activity abroad in the first place. And in doing so Congress would run up against separation of powers issues, because in our system, it's not really Congress's place to dictate to the President how he carries out foreign security activities.

[1] I think as it is it might be a 4th amendment violation, but it's not contrary to any statute that I'm aware of, at least not any American statute.