First, this would show support for international laws against torture and help put the lid back on Pandora's box. This would directly reduce the probability of captured U.S. citizens being tortured. No, that probably does not include those captured by ISIL, but are rogue terrorist organizations all U.S. citizens will need to worry about both now and in the future?
Second, it would close a dangerous precedent. Prosecuting those who committed illegal acts, even though they were assured of immunity, sends a clear message that individuals are still responsible for their actions. Without this sense of individual responsibility U.S. agencies will be capable of utterly anything and, the way things work, capability becomes reality more often than not. Most Americans would sleep better knowing that the NSA's workers are accountable to the law rather than immune to it if their superiors say so, as is currently the case due to the precedent set by the CIA.
Third, the implications for future diplomacy are a nightmare if the U.S. does nothing. There will be no moral high ground for the U.S. to stand on if the U.S. refuses to seek justice.
Finally, it's the right thing to do. The U.S. could set an example for others to follow. To do nothing does precisely the opposite.