Odd way for Congress to write a law. The law flat out says "Use only interrogation techniques in Army Field Manual". So what does this mean really? If Congress intends to outlaw waterboarding, why not just say "no waterboarding?"
Law, acts of Congress, ought to be declarations of general principles that don't change over time. The Army rewrites its regulations and field manuals nearly every year. Next year's edition might permit much harsher treatment. Does Congress want to delegate interrogation policy to a board of Army officers? A law saying "Use this manual" isn't general or long lasting.
Of course, a law that defines acceptable and unacceptable interrogation procedures in 25 words or less is hard to write. On the other hand Congress has thousands of bright lawyers in it and working for it and you would think they could do better than this.